Musnad Abu Ya'la al-Mawsili 113:33

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Ismail narrated to us, from Khalid al-Hadhdha’, from Abu Qilabah, from Anas, who said: "Seven [nights] for the virgin and three for the previously married woman. If I were to say that it is from the Messenger of Allah, may Allah bless him and grant him peace, I would be truthful, but it is the Sunnah."