Musnad Abu Ya'la al-Mawsili 115:224

Ubayd Allah ibn Umar al-Qawariri narrated to us, Yazid ibn Zuray' narrated to us, Hajjaj al-Ahwal al-Bahili narrated to us, from Qatadah, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about a man who sleeps through the prayer or is heedless of it. He said: "Its expiation is that he prays it when he remembers it."