Musannaf Ibn Abi Shayba 3:2569

Abu Khalid al-Ahmar narrated to us, from Yahya ibn Sa'id, from 'Ubayd Allah ibn 'Abd Allah, from his father, who said: I came to 'Umar while he was praying, so he placed me on his right side. Then Yarfa' came, so he placed us behind him.