How did 'Umar reconcile accepting the report of 'Abd al-Rahman with requesting three from Mu'adh?

The questioner : Shaykh, regarding this, how can we reconcile 'Umar's (may Allah be pleased with him) acceptance of a single report, that of 'Abd al-Rahman, with his request for three from Mu'adh when three came to his door?

The Shaykh : Look at Abu Musa.

The questioner : Trustworthy Abu Musa, I am bringing a supporting narration.

The Shaykh : Yes. First, in this instance, 'Umar (may Allah be pleased with him) saw that 'Abd al-Rahman did not need corroboration. This is because 'Abd al-Rahman ibn 'Awf was one of the ten promised Paradise, and also because what he reported agreed with what these select individuals had agreed upon. This was evidence that it was the truth, and an indication that his statement was true. As for the case of Abu Musa, you know that Abu Musa reported this hadith to defend himself, because 'Umar wanted to punish him had he not brought the hadith.
A person who is defending himself may understand a matter in a way other than its intended meaning; he may understand a matter in a way other than its intended meaning. So 'Umar (may Allah be pleased with him) wanted to be cautious in this matter so that no doubt remained, because reporting is like testimony in some respects. This is Abu Muslim; when he reported about the Prophet (peace and blessings be upon him), and although it is very far-fetched that he would intentionally lie for his own benefit, given the status of the Companions (may Allah be pleased with them), he might have understood the text contrary to what the Prophet (peace and blessings be upon him) intended, and he might have acted according to his understanding of the text, which was contrary to what the Messenger (peace and blessings be upon him) intended. So 'Umar wanted to be cautious in this matter.