The Messenger warned people not to lie about him: "Those who lie about me should prepare their abodes in the Fire"  and: "Whoever relates from me falsely is a liar."  In the face of such warnings, would the Companions, who had sacrificed their entire lives for the cause of Islam, even think of lying about the Messenger?
Based on these considerations, the Companions took great care when narrating Traditions so that no mistake or misunderstanding would occur. For example 'Ali, the Prophet's cousin and son-in-law who was always with him, used to say: "I fear to narrate a Tradition from the Messenger so much that I would rather fall from Heaven than speak a lie on his behalf." 
'Abd Allah ibn al-Mas'ud, one of the most knowledgeable and nearest Companions, was similarly careful. When asked to report from the Messenger, he began with: "The Messenger of God said," stopped and bowed his head, breathed deeply and unbuttoned his collar while his eyes filled with tears. After the narration, he added: "The Messenger of God said this, or something like this, or something more or less like this." 
Zubayr ibn 'Awwam, one of the ten Companions assured Paradise, narrated only a few Traditions from the Messenger. When his son asked him why, he replied: "I am so afraid that I might say something contrary to what the Messenger really said. For he declared: 'Those who lie about me intentionally should prepare their abodes in the Fire.'"  Anas ibn Malik, who served the Messenger for 10 years, said: "If I were not so afraid of making a mistake, I would relate many more narrations from the Messenger." 
'Abd al-Rahman ibn Abi Layla met 500 Companions. When he visited a place, people would say: "The man who met 500 Companions has come to our town." He had a great influence on Abu Hanifa and Imam Abu Yusuf. He reports: "I was personally familiar with 120 Companions. Sometimes all of them were in the same mosque. When they were asked about something, each would wait for the other to answer. If they were asked to narrate a Tradition, no one would dare to. Finally, one of them would place his trust in God and begin to narrate. He would always add: 'The Messenger said this, or something like this, or something more or less like this.'" 
Zayd ibn Arqam was one of the first people to embrace Islam. In the early days of Islam, the Messenger would meet with the Muslims secretly in his house. Zayd was appointed superintendent of the public treasury during the caliphates of 'Umar and 'Uthman. When he saw 'Uthman give items from the treasury to his relatives, he told him: "O Commander of the Faithful. People will suspect me and will no longer trust me. Allow me to resign." When 'Abd al-Rahman ibn Abi Layla asked him to narrate a Tradition, Zayd answered: "My son, I have become old and forgetful. Narrating about the Messenger is not something easy." 
 Muslim, "Muqaddima," 1.
 Bukhari, "Istitaba," 6; Abu Dawud, "Sunna," 28.
 Ibn Ma'ja, "Muqaddima," 3.
 Bukhari, "'Ilm," 38; Muslim, "Zuhd," 72.
 Darimi, "Muqaddima," 25.
 Dhahabi, Siyaru A'lam al-Nubala', 4:263.
 Ibn Ma'ja, "Muqaddima," 3.