Musnad Ahmad 1163:14

Sufyan narrated to us, from Ibn al-Asamm, [Abdullah said:] My father said: "And it was read to Sufyan that his name is Ubaydullah ibn Abdullah, the nephew of Yazid ibn al-Asamm," from his paternal uncle, from Maymunah—and she is his maternal aunt—who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would prostrate, and there was a lamb there that wanted to pass between his hands, he would spread his arms apart."