Musannaf Ibn Abi Shayba 16:371

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, from Ibn Sirin, who said: Ibn ‘Umar used to command the cleaning of the spaces between the teeth (al-ikhlal), and he would say: "Indeed, if that is neglected, it weakens the molars."