Musnad Abu Ya'la al-Mawsili 42:217

Zuhayr narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from 'Amir—Abu Khaythama al-Ahwal said—from al-Hasan, from Abu Sa'id al-Khudri, from the Prophet (may Allah bless him and grant him peace), regarding the one who forgets the prayer: He said: "He shall pray it when he remembers it."