Sahih Muslim Book 4, Hadith Number 1006.

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.

Musa b. Talha reported it on the authority of his father: The Messenger of Allah (may peace be upon him) said: When one of you places in front of him something such as the back of a saddle, he should pray without caring who passes on the other side of it.