Musannaf Ibn Abi Shayba 1:1127

Ibn al-Mubarak narrated to us, from Yunus, from al-Zuhri, who said: Urwah informed me, from his father, that Abu Bakr al-Siddiq said while he was addressing the people: "O assembly of Muslims, be shy of Allah. For by the One in whose hand is my soul, I remain, when I go to the privy in the open space, covering my head out of shyness from my Lord."