Musannaf Ibn Abi Shayba 9:1610

Abu Bakr narrated to us, he said: Hafs narrated to us, from Abdullah ibn Muslim, from Humaydah, a freedwoman of Umar ibn Abd al-Aziz, who said: Umar used to say: "Do not let my daughters sleep lying on their backs, for the devil continues to covet them as long as they remain in that state."