It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I prayed with the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) said the takbir when he began the prayer and raised his hands, and when he intended to bow (for ruku') he raised his hands, and also after raising his head from the bowing (he did the same)."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18247