All of the Companions of Allâh's Messenger (peace and blessing be upon him) are trustworthy, based on Allâh's testimony, and due to His praise as well as the praise of His Messenger (peace and blessing be upon him). An-Nawawî said in At-Taqrîb, which As-Suyûtî explained in Tadrîb Ar-Râwî:
“All of the Companions are trustworthy - those who were involved in the fitan [the war which took place between them] and those other than them - according to the consensus of the scholars.”
Al-Hâfidh Ibn Hajr said in Al-Isâbah:
“Ahl-us-Sunnah have unanimously agreed that all of them (the Sahâbah) are trustworthy, and no one opposed this except some deviants amongst the innovators.”
This is why there is no harm in not knowing the identity of a Companion in the chain of narration of a hadeeth. If a Tâbi'î says: “On the authority of a man who accompanied the Prophet (peace and blessing be upon him)...” this does not affect the report. This is because not knowing the identity of a Companion (in the chain of narration) causes no harm, since they are all trustworthy. Al-Khatîb Al-Baghdâdî said in his book Al-Kifâyah:
“Every hadîth that has a chain of narration that is connected between the one who reported it and the Prophet (peace and blessing be upon him) is not to be acted upon until first having affirmed the trustworthiness and reliability of its men (in the chain). It is an obligation to investigate into their condition, except a Sahâbî (Companion) who raises it (or attributes the hadîth) to the Prophet (peace and blessing be upon him). This is because the trustworthiness of the Companions is established and well known from Allâh's testimony, and His informing us of their pure state and His preference of them in the texts of the Qur'ân…”
Then he continues to list some ayât and ahâdîth concerning their virtue and then says,
“And even if Allâh or His Messenger (peace and blessing be upon him) had not mentioned anything about them from what we just stated now (of their virtues), their Hijrah, Jihâd, offering of support, sacrificing of life and wealth, experience of the killings of their parents and children, mutually advising for the Religion, their strong Faith and unshakeable Certainty, (all of this) would necessitate their trustworthiness and having a firm belief in their honesty and integrity. It would necessitate that they are the best of those that receive testimony and approval of their trustworthiness and reliability from all people that come after them forever and ever.”
Then he reports a narration with the chain connected to Abu Zur'ah, who said:
“If you see a man seeking to belittle anyone amongst the Companions of Allâh's Messenger (peace and blessing be upon him), then know that he is a heretic (zindîq). This is because according to us the Messenger of Allâh (peace and blessing be upon him) is true and the Qur'ân is true, and the only ones who brought this Qur'ân and the Sunan (plural of Sunnah) to us are the Companions of Allâh's Messenger (peace and blessing be upon him). They only wish to disparage and demolish our witnesses so that they can nullify the Book and the Sunnah, whereas they are more deserving of being disparaged for they are the heretics (zanâdiqah).”