Musnad Ahmad 687:18

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Asim ibn Kulayb, from his father, from Wa’il al-Hadrami, who said: "I prayed behind the Messenger of Allah (peace and blessings of Allah be upon him). He said the takbir when he entered [the prayer] and raised his hands. When he intended to bow, he raised his hands. When he raised his head from bowing, he raised his hands. He placed his palms [on the ground], kept his elbows away from his sides, and spread his left thigh from his right [thigh], and he gestured with his index finger."