Musnad Ahmad 1016:5

Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Amr ibn al-Harith narrated to us, from Ja'far ibn Rabi'ah, from Ibn Hurmuz, from Ibn Buhaynah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to, when he prostrated, spread his arms wide in his prostration until the whiteness of his armpits could be seen.