Musnad Abu Ya'la al-Mawsili 129:28

Musa ibn Muhammad ibn Hayyan narrated to us, Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from Anas, who said: The prayer was called, and there was something between the Messenger of Allah (may Allah bless him and grant him peace) and his wives. He began to turn some of them away from others. Abu Bakr (may Allah be pleased with him) came and said: "Throw dust into their mouths and go out to the prayer."