Musnad Ahmad 32:1048

Bahz narrated to us, Hammam narrated to us, from Qatadah; and Affan narrated to us, he said: Yazid ibn Ibrahim narrated to us, Qatadah narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Maintain moderation in prostration, and let not one of you stretch out his forearms like the dog, and let him not spit in front of him, nor to his right, for he is conversing with his Lord; but [let him spit] to his left or under his foot."