Musnad Abu Ya'la al-Mawsili 133:11

Zakariya ibn Yahya narrated to us, Dawud ibn al-Zibriqan narrated to us, Ali ibn Zayd narrated to us, from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, who said: "The likeness of the five prayers is like a fresh, flowing river, or a deep body of water, at the door of one of you, in which he bathes five times every day. Does any of his filth remain upon him?"