Sahih Muslim Book 4, Hadith Number 1034.

Chapter : The excellence of the prayer and the way it is begun and the excellence of Ruku’ and moderation in it and prostration and moderation in it, etc.

Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: A woman, an ass and a dog disrupt the prayer, but something like the back of a saddle guards against that.