It is known that all the Companions are trustworthy, and trustworthiness is justice combined with precision; as for justice, it is certainly established in them without doubt; so how can their precision be known?

The questioner : The fourth question: It is known that the Companions (may Allah be pleased with them) are trustworthy, and trustworthiness is justice combined with precision. We guarantee the justice of the Companions, but who guarantees their precision for us?

The Shaykh : The one who guarantees the precision of the Companions for us is the same one who guarantees the precision of those after the Companions. This is the concise answer, but it seems to me that it requires some detail. The point is that we must know the method by which scholars arrive at knowing the precision of narrators in general, and the precision of the Companion narrators in particular. I say: the method is one, but it is strange that the question remains directed at: how do we know the precision of the Companions? The answer: There are two ways to know the precision of the Companions and to know the precision of the Tabi'un and those after them. This path is confined to two aspects. The first aspect is if the knowledgeable person is able to have personal contact with the narrator, then by virtue of his companionship with the narrator, it becomes clear to him, closely and intimately, whether he is a precise memorizer or not. For example, if a Tabi'i met Abu Hurayrah and heard many of his hadiths, especially since his hadiths do not differ or become confused, no matter how many gatherings there were or how far the distances were. As mentioned in his biography, one of the Umayyad califs asked him to narrate hadiths in his presence, and he instructed one of his scribes to write down what Abu Hurayrah was narrating secretly and discreetly. Then, after a year, he asked him to narrate what he had narrated a year before, and instructed the scribe to take the hadiths and compare them with what he heard again from Abu Hurayrah. They were greatly astonished that they did not find that Abu Hurayrah had omitted even a single letter.
This is the first aspect by which scholars are able to know the precision of the narrator, which is personal contact. Then, the information that reaches these people who were able to have direct contact with the narrators, whether they were from the Companions or the Tabi'un, becomes a narration that is transmitted to those after them until it is recorded in the books of narrators. The other path is by which its owner can know the precision of the narrator, even if there are years between them. This path is examining his hadiths, tracking them, and comparing them with the hadiths of the majority of narrators. If it becomes clear to him that these hadiths have no contradiction or conflict in their narration with the other hadiths, it appears that this narrator is a precise memorizer.
This path and that one are the two paths by which the scholars of hadith arrive at knowing the precision of the narrator, and there is no difference in that whether this narrator is a Companion, a Tabi'i, or a Tabi' al-Tabi'in.