Sahih Muslim Book 4, Hadith Number 1029.

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.

Sahl b. Sa’d al-Si’idi reported: Between the place of worship where the Messenger of Allah (may peace be upon him) prayed and the wall, there was a gap through which a goat could pass.