Musannaf Ibn Abi Shayba 3:1569

Yahya ibn Sa'id narrated to us, from Thawr, from Khalid ibn Ma'dan, from Abu Ziyad, the freed slave of the family of Darraj: I saw, and I have not forgotten—for I have not forgotten—that Abu Bakr, when he stood in prayer, would say like this, and he placed his right [hand] upon his left.