Sahih Muslim Book 4, Hadith Number 1007.
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 on the authority of his father: We used to say prayer and the animals moved in front of us. We mentioned it to the Messenger of Allah (may peace be upon him) and he said: If anything equal to the back of a saddle is in front of you, then what walks in front, no harm would come to him. Ibn Numair said: No harm would come whosoever walks in front.