Usayd's mother said: 'I said to Abu Qatadah: "Why do you not narrate the Prophet's hadiths as other people do?" He said: "I heard God's Messenger (peace be upon him) say: 'Whoever attributes something false to me prepares for himself a position in hell to recline upon'. As the Prophet said this, he smoothed the ground with his hand".' [Bukhari]
This hadith states why many of the Prophet's companions were especially reluctant to quote him, fearing that they might replace a word here or there, or misquote him in some other way. If they were to attribute it to the Prophet and state that he said this, they feared that it might be counted as a lie. This would, then, put them in the position the Prophet warned against. Needless to say, God knows that they intended no such a thing, and if they erred, it would have been a genuine mistake. We know that God would not punish anyone for a genuine mistake, as the Prophet himself made clear. Nevertheless, these companions remained reluctant to quote the Prophet for fear of making a mistake. Some of them would quote him only very sparingly, as in the case of Abu Qatadah, who would have reported several times as many hadiths as are related through him. Others would qualify any quotation they made from the Prophet by adding the phrase, 'or he might have said something similar to this'.
It is often the case that the Prophet's companions might say a hadith which they heard the Prophet say, but without attributing it to him. This means that the hadith would be reported as if it was said by the companion reporting it, but scholars of Hadith would know that no one of the Prophet's companions would ever have said anything relevant to the religion of Islam unless he had heard it from the Prophet. This is one of the reasons why Hadith scholars include such reports, calling them athar, particularly when a report involves a prohibition. No one would dare describe anything as forbidden without clear evidence from the Quran or the Sunnah, because the authority to forbid anything belongs only to God.
"Al-Adab al-Mufrad with Full Commentary: A Perfect Code of Manners and Morality" - Adil Salahi