Musannaf Ibn Abi Shayba 33:1300

Abu Khalid al-Ahmar narrated to us, from Dawud, from Abu Nadra, from Abu Sa’id, the freed slave of Abu Usayd, who said: When ‘Umar would pray, he would have the people exit the mosque, and he would head toward us. When he saw his companions, he dropped the whip and sat down. He said, "Supplicate." So they supplicated. He said: He continued to supplicate and supplicate until the turn for supplication reached me, and I supplicated while I was a slave. I saw him supplicate and weep with a weeping that a bereaved mother does not weep. I said to myself, "This is the one whom you say is harsh."