In addition, the Traditionists were extremely sensitive to lying. If it could be proven that a narrator had lied even once during his or her life, all Traditions coming from that source were rejected. Narrators had to be completely truthful, have a keen memory, be very careful in practicing Islam, and not be involved in sectarianism. Moreover, if reliable narrators became forgetful or had similar mental difficulties, their Traditions were no longer accepted. For example, when Ibn Abi Lahi'a, famous for austerity and God-consciousness, lost the notebook from which he used to relate Traditions, Imam Bukhari restricted himself to those of his narrations confirmed or reinforced by other reliable narrators.
It is said that one's literary style is identical with that particular person. In other words, if you are a careful reader, you can identify an author by his or her style and distinguish him or her from others. Traditionists dedicated themselves to Hadith, and were so familiar with the field that they could distinguish easily between the Prophet's sayings and those of everyone else, no matter how gifted.
Another way was to judge them according to the Qur'an and the mutawatir hadith. If three or more Companions reported a hadith from the Prophet, which was then handed down by several transmission chains of reliable narrators, it is mutawatir. Traditions reported from the Prophet by one Companion are called ahadi. Such Traditions usually were accepted as authentic after judged according to the Qur'an and mutawatir Traditions.
Although not an objective method, some saintly scholars saw the Messenger while awake and received directly from him. The hadith qudsi: "I was a hidden treasure. I wished to be known, and so created the universe" is reported to belong to this class.  Jalal al-Din al-Suyuti is reported to have met with the Messenger several times while awake. Before writing down a hadith he considered authentic, Imam Bukhari performed wudu' and referred it to the Messenger. He recorded the hadith in his notebook only after receiving the Messenger's approval.  Some Traditionists saw the Companion who had narrated the hadith from the Prophet.
The Traditionists wrote multi-volume works about narrators. Included in such accounts was the various narrators' life histories: where and when they were born, where they emigrated and lived, their teachers, from whom they received and to whom they narrated Traditions, and when and where they died. The first book of this genre was 'Ali ibn al-Madini's Kitab al-Ma'rifat al-Sahaba (The Book of Knowledge about the Companions). Among the most significant are the following: Ibn 'Abd al-Barr's Al-Isti'ab fi Ma'rifat al-Ashab (The Comprehensive Book of Knowledge about the Companions), Ibn Hajar al-Asqalani's Al-Isaba fi Tamyiz al-Sahaba (Finding the Truth in Judging the Companinons), Ibn al-Athir's Usd al-Ghaba (The Lions of the Forest), Ibn Sa'd's Al-Tabaqat al-Kubra' (a most comprehensive biographical dictionary of the leading Companions and of the Tabi'un scholars), and Tarikh Ibn Asakir (History by Ibn Asakir), Tarikh al-Bukhari (History by Bukhari) and Yahya ibn Ma'in's Al-Tarikh al-Kabir (The Great History).
The greatest Traditionists, among them Bukhari, Muslim, Tirmidhi, Abu Dawud, Nasa'i, Ibn Maja, and Ahmad ibn Hanbal, collected authentic Traditions in voluminous books. Others, such as Maqdisi, collected fabricated Traditions; still others, who came later, tested once more the authenticity of all previously collected Traditions. Ibn al-Jawzi (d. 597 ah) judged several Traditions in Ibn Hanbal's Musnad to be either weakly transmitted or fabricated, although Ibn al-Jawzi himself belonged to Ibn Hanbal's legal school. Later, Ibn Hajar al-Asqalani made a detailed examination of these very same Traditions and, with the exception of thirteen, proved their authenticity. Jalal al-Din al-Suyuti (d. 911 ah) scrutinized those Traditions once more and concluded that none were fabricated, although a few may have weak chains of transmission. Suyuti also reviewed Ibn al-Jawzi's Al-Mawdu'at al-Kubra' (A Great Collection of Fabricated Traditions) and sorted out the authentic ones. Thinking that the rest might not be fabricated either, he wrote Al-Laa'li al-Masnu'a (The Artifical Pearls).
Other great Traditionists compiled additional compendia. Such leading Traditionists as Bukhari and Muslim, tremendously exacting scholars, did not include many Traditions in their collections. Hakim's Al-Mustadrak 'ala al-Sahihayn (Addendum to The Two Collections of Authentic Traditions) is a voluminous appendix to Bukhari and Muslim. It was reviewed closely by Hafiz Dhahabi, who was famous for his keen memory.
In later centuries, books were written on widespread maxims, wise sayings, or proverbs that are regarded as Hadith. Sakhawi's Maqasid al-Hasana and Ajluni's Kashf al-Khafa' examined them one by one and explained which are truly Traditions and which are not. For example, apart from many authentic Traditions and Qur'anic verses encouraging people to learn, such popular sayings as: "Seek knowledge from the cradle to the grave" and "Seek knowledge even if it is in China" were tested by the Traditionists and shown not to be real Traditions.
After such tremendous studies, detailed examinations, and exacting verifications, we can state that the collections of authentic Traditions no longer contain fabricated Traditions. Those who continue to question the Traditions and Sunna act out of nothing more than religious, political, and ideological prejudice, as well as from biased Orientalist scholarship, to cast doubt on this vital source of Islam and its implementation in one's daily life.
 Ibn Hajar, Tahdhib al-Tahdhib, 9:49.