Prayer (Kitab Al-Salat): Details of Commencing Prayer
Dawud :: Book 3 : Hadith 901
Narrated AbuHurayrah:
The Companions of the Prophet (peace_be_upon_him) complained to the Prophet (peace_be_upon_him) about the hardship when they kept their forearms far away from their sides while prostrating. He said: Take help with the elbows (by spreading them on the ground and sticking them to the sides).