Musnad Abu Ya'la al-Mawsili 115:12

Kamil ibn Talhah al-Jahdari narrated to us, Hammad ibn Salamah narrated to us, from Qatadah, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "Maintain moderation in prostration, and let not one of you spread out his forearms like the dog."