Musannaf Ibn Abi Shayba 34:14

Ibn ‘Ulayyah narrated to us, from Muhammad ibn Ishaq, from a man, from ‘Abd al-Rahman ibn Ka‘b ibn Malik, who said: I was my father’s guide when his eyesight was gone. Whenever I went out with him to the Friday prayer and he heard the call to prayer, he would seek forgiveness for Abu Umamah As‘ad ibn Zurarah and pray for him. I said to him, "O my father, what is the matter with you that whenever you hear the call to prayer on Friday, you seek forgiveness for Abu Umamah, pray for him, and invoke blessings upon him?" He said, "O my son, he was the first to lead us in the Friday prayer before the arrival of the Messenger of Allah (may Allah bless him and grant him peace), at Baqi‘ al-Khadamat, in the valley of Banu Bayadah." He said, "And how many were you on that day?" He said, "We were forty men."