Musnad Ahmad 27:940

Sakan ibn Nafi' al-Bahili Abu al-Husayn narrated to us, Salih ibn Abi al-Akhdar narrated to us, from al-Zuhri, from Salim ibn Abd Allah, from his father, who said: "I was a young, unmarried man, and I used to sleep in the mosque during the time of the Messenger of Allah, may Allah bless him and grant him peace. Dogs would come and go in the mosque, and they would not sprinkle anything upon any of that."