Sahih Bukhari
Times of the Prayers
Volume: 001 / Book Number: 010 / Hadith Number: 509
Narrated Anas:
The Prophet said, "Do the prostration properly and do not put your fore-arms flat with elbows touching the ground like a dog. And if you want to spit, do not spit in front, nor to the right for the person in prayer is speaking in private to his Lord."