Musannaf Ibn Abi Shayba 3:62

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from ‘Asim ibn Kulayb, from Muharib ibn Dithar, from Ibn ‘Umar, he said: I saw him raising his hands in the bowing (ruku‘) and the prostration (sujud). So I said to him: "What is this?" He said: "The Prophet, may Allah bless him and grant him peace, when he would stand up from the two rak‘ahs, he would say 'Allahu Akbar' and raise his hands."