A woman once asked Caliph Abu Bakr if she could inherit from her grandchildren. He answered: "I have seen nothing in the Qur'an that allows this, nor do I remember the Messenger saying anything on this point." Mughira ibn Shu'ba stood up and said: "The Messenger allowed the grandmother to receive one-sixth (of the estate)." Abu Bakr asked Mughira if he could produce a witness to testify to this. When Muhammad ibn Maslama testified to it, Abu Bakr gave the woman one-sixth of her grandson's estate.
When the Messenger declared: "Those called to account for their deeds on the Day of Judgment by God will be ruined," 'A'isha asked: "What about the Divine declaration in the Qur'an: Then they will be called to account (for their deeds), and it will be an easy act of giving account'? The Messenger answered: "It is about presentation. Everyone will give account to God for their deeds. If those who did evil deny their evil deeds, God will inform them of their deeds. Such people will be ruined." 
As recorded in Bukhari, 'Umar narrates:
I heard Hisham ibn Hakim pronounce some words of Surat al-Furqan somewhat differently from the way the Messenger taught me. I waited patiently until he had finished praying, and then asked him: "Who taught you such a recitation?" When he told me that he had learned it from the Messenger, I took him to the Messenger and explained the situation. The Messenger asked Hisham to recite the sura, which he did. The Messenger nodded, saying: "This is the way it was revealed to me." Then he asked me to recite, which I did. Again he nodded and said: "Thus it was revealed." He added: "The Qur'an is revealed in seven different ways. Recite it in the way easiest to you."
The Companions were so devoted to the Sunna that they would travel long distances to learn just one hadith. For example, Abu Ayyub al-Ansari traveled from Madina to Egypt to check one hadith's exact wording. Among those who had received it from the Messenger, only 'Uqba ibn Amir was still alive and living in Egypt. Abu Ayyub arrived in the capital city and, calling on its governor Maslama ibn Mukhallad, found a guide to take him to 'Uqba. When he found this Companion in a street, he asked him about: "Whoever covers (hides) a believer's defect in the world, God will cover his (or her) defects in the Hereafter." Being told by 'Uqba that his memory was correct, Abu Ayyub took his leave, saying: "I came just to ask about this hadith. I wouldn't like to make my intention impure [by staying] for some other reason." 
As related in Bukhari, Jabir ibn 'Abd Allah traveled for a whole month just to receive a hadith directly from its narrator, 'Abd Allah ibn Unays. Finding 'Abd Allah, he said: "I've been informed that you relate a hadith that I didn't hear from the Messenger. Fearing that one of us may die before I learn it, I have come to you." Jabir learned the hadith and returned to Madina.
Such journeys continued throughout the following centuries. Sa'id ibn al-Musayyib, Masruq ibn Ajda, and others made long journeys to learn a single hadith or even to confirm a single letter of one hadith. Kathir ibn Qays relates that one such lover of knowledge traveled from Madina to Damascus to learn a single hadith from Abu al-Darda'. The Tabi'un exhibited the same degree of caution as the Companions when narrating a Tradition. As stated by A'mash, they would prefer the sky to collapse on them than to add so much as a wrong vowel to a hadith. 
The Ahl al-Sunna wa al-Jama'a agree on the Companions' absolute truthfulness. However, after internal conflicts broke out among the Muslims, the Tabi'un began to scrutinize whatever hadiths they heard and to inquire about their narrators' truthfulness. Muhammad ibn Sirin says: "Before, we didn't ask about the narrators. But after the internal conflicts broke out, we began to ask." 
People of weak character and ungrounded faith fabricated Traditions to promote their sectarian beliefs. The Nasiba (the Umayyads and their supporters who opposed 'Ali) forged Traditions in favor of 'Uthman and Mu'awiya and against 'Ali, and the Rafidites (Shi'a extremists) forged Traditions against 'Uthman and Mu'awiya and for 'Ali. This caused meticulous, truth-seeking scholars to undertake a detailed and careful examination of each reported hadith and its narrators' character. Abu al-'Aliya says: "We were no longer content with what was reported to us from a Companion. We traveled to receive it directly from the Companion or Companions who had narrated it, and to ask other Companions who knew about it."
Imam Muslim relates that Bushayr al-'Adawi narrated a hadith to Ibn 'Abbas. Noticing that the latter was not paying attention to him, Bushayr asked in surprise: "Why aren't you listening to me? I'm narrating a hadith." Ibn 'Abbas answered: "In the past, our hearts would jump for joy and excitement when somebody began to narrate a hadith, saying: 'The Messenger said.' We would be fully attentive. But after people began to travel from place to place, we only receive from those whom we already know." 
Ibn 'Abd al-Barr, the great scholar of Muslim Spain (Andalusia), reports from Amir ibn Sharahil al-Sha'bi, one of the greatest Tabi'un scholars: Rabi' ibn Husayn related to Sha'bi the hadith: "Those who recite ten times: 'There is no god but God, One, and He has no partner. His is the kingdom, and His is all praise. He gives life and causes death. He is powerful over everything,' may earn as much reward as those who free a slave." Sha'bi asked Rabi' who had narrated that hadith to him. He said that 'Abd al-Rahman ibn Abi Layla' had done so. Sha'bi then left and found Ibn Abi Layla, who was living in another city. Ibn Abi Layla testified to the hadith's authenticity, saying he had heard it from Abu Ayyub al-Ansari. 
Such great scholars as Ibn Shihab al-Zuhri, Ibn Sirin, Sufyan al-Thawri, Amir ibn Sharahil al-Sha'bi, Ibrahim ibn Yazid al-Naha'i, Shu'ba, Abu Hilal, Qatada ibn Di'ama, Hisham al-Dastawa'i and Mith'ar ibn Qudam did their best to determine which Traditions were authentic and which were fabricated. When they were unsure of a Tradition's authenticity, they would consult each other. For example, Abu Hilal and Sa'id ibn Abi Sadaqa asked Hisham al-Dastawa'i about one Tradition's exact wording just to be sure. Shu'ba and Sufyan al-Thawri referred to Mith'ar a matter about which they did not have exact knowledge. Such great scholars did not allow fabricated Traditions to spread. Whenever and wherever they heard people known for their sectarian views narrate a Tradition, these Traditionists would ask who had related this Tradition to them.
Those truth-loving and truth-seeking scholars did not refrain from revealing the weaknesses of their families or relatives. For example, Zayd ibn Unaysa warned Traditionists not to receive hadith from his brother, perhaps because of his forgetfulness, carelessness, or sectarianism. When asked about his father, 'Ali ibn al-Madini, the first to write on the Companions, answered: "Ask others about him." When they insisted, he explained: "Hadith means religion. My father is weak on this point."
Waki' ibn Jarrah, who was brought up in the school of Abu Hanifa and was a tutor of Imam Shafi'i, said: "As far as I know, I have never forgotten anything once I heard it. Nor do I remember anything that I had to repeat in order to memorize, if I only heard it once." Despite his keen memory, Imam Shafi'i once complained to Waki' about his poor memory. Waki' answered: "Refrain from sin. Knowledge is a light from God, and so cannot be granted to sinful people." When his father Jarrah was narrating a hadith, Waki' was always nearby. When asked why, he answered: "My father works in the state's finance department. I am afraid he might soften some Traditions in favor of the government. I accompany him to prevent such a lapse." 
While the Traditions were being written down, they also were being memorized by some of the greatest Traditionists of Islamic history. For example, Ahmad ibn Hanbal memorized around one million Traditions, including authentic, good, weak, and fabricated ones (some were identical in text but had different narration chains). His Musnad contains only 40,000 Traditions out of 300,000 Traditions. Yahya ibn Ma'in memorized both authentic and fabricated Traditions. When Ibn Hanbal asked him why he did so, he replied: "I inform people of fabricated Traditions so they may choose the authentic ones." Many other scholars engaged in this activity and knew hundreds of thousands of them by heart. Among them, the most famous are Zuhri, Yahya ibn Sa'id al-Qattan, Bukhari, Muslim, Daraqutni, Hakim, Dhahabi, Ibn Hajar al-'Asqalani, and Imam Suyuti.
Thanks to the tremendous efforts of such Traditionists, authentic Traditions were distinguished from fabricated ones. In addition to recording authentic Traditions in volumes and memorizing them, many Traditionsits wrote volumes on the narrators' character so people would know who was reliable or unreliable, careful or careless, profound and meticulous or superficial, and God-fearing or heedless. When people warned them that revealing the defects of others brings shame upon those people, the scholars would answer: "Hadith means religion. Therefore it should be given greater care than the hiding of the narrators' defects." Yahya ibn Sa'id al-Qattan, renowned for being alert to sins, used to say: "In the presence of God, I would rather have them as enemies than the Messenger."
 Khatib al-Baghdadi, "al-Rihla fi Talab al-Hadith," 118–24.
 Khatib al-Baghdadi, al-Kifaya fi 'Ilm al-Riwaya, 178.
 Muslim, "Muqaddima," 5.
 Muslim, "Muqaddima," 5.
 M. 'Ajjaj al-Khatib, op. cit., 222.
 Ibn Hajar, ibid., 6:84.