Musnad Ahmad 32:32

Ishaq ibn Yusuf al-Azraq narrated to us, from Ibn Abi Arubah, and Yazid ibn Harun, who said: Sa'id informed us, from Qatadah, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever forgets a prayer or sleeps through it, then its expiation is that he prays it when he remembers it." Yazid said: "Then its expiation is that he prays it when he remembers it."