Musannaf Ibn Abi Shayba 21:574

Ubayd Allah ibn Musa narrated to us, from Isra'il, from Ash'ath ibn Abi al-Sha'tha', from a man from the Ansar who narrated to him from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) passed by him while he was supplicating with his hands. He said, "Make it one, for indeed He is One."