This document describes a man from the Ansar tribe who was friends with Umar. When Umar was not present with the Prophet Muhammad, the man from Ansar would be and would tell Umar what he heard from the Prophet. And when the man from Ansar was absent, Umar would be with the Prophet and tell the man what he heard.

2.  Narrated 'Umar: There was a man from the
Ansar (who was a friend of mine). If he was not
present in the company of Allah's Apostle I used
to be present with Allah's Apostle, I would tell
him what I used to hear from Allah's
Apostle, and when I was absent from Allah's
Apostle he used to be present with him, and he
would tell me what he used to hear from Allah's
Apostle .
(Sahhih Al Bukhari, Vol. 9, Hadith No. 362)