Musannaf Ibn Abi Shayba 3:6469

Abu Bakr narrated to us, he said: Abu Usama narrated to us, he said: Abu Hilal narrated to us, he said: Humayd ibn Hilal narrated to us, from Abu Burdah, that Abu Musa al-Ash'ari entered upon his sister while she was prostrating without bowing (ruku'), and he did not find fault with her for that.