Musnad Ahmad 27:1868

Muhammad ibn Fudayl narrated to us, from ‘Asim ibn Kulayb, from Muharib ibn Dithar, who said: I saw Ibn ‘Umar raising his hands whenever he bowed, and whenever he raised his head from bowing. He said: So I said to him: "What is this?" He said: "The Prophet, may Allah bless him and grant him peace, when he stood up for the two rak'ahs, would say the takbir and raise his hands."