Musannaf Ibn Abi Shayba 3:5278

Waki’ narrated to us, [saying] Shu’bah narrated to us, from al-A’mash, from Abu Sufyan, from ‘Ubayd ibn ‘Umayr, [who] said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The likeness of the five prayers is like a flowing river at the door of one of you, in which he bathes five times every day. So what does that leave of [his] filth?"