Musnad Abu Ya'la al-Mawsili 115:268

Zuhayr narrated to us, Ishaq bin Yusuf al-Azraq narrated to us, Sa'id bin Abi 'Arubah narrated to us, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever forgets a prayer or sleeps through it, then its expiation is only that he prays it when he remembers it."