Musnad Abu Ya'la al-Mawsili 129:72

Muhammad ibn Ishaq al-Musayyibi narrated to us, Abdullah ibn Nafi’ narrated to me, from Abdullah ibn ‘Umar, from Humayd al-Tawil, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) said: "For the virgin, seven [nights], and for the previously married woman, three [nights]."