Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1454
Qatada reported from Anas b. Malik that the Messenger of Allah (may peace be upon him) said: He who forgets the prayer should say it when he remembers it, there is no explation for it, except this. Qatada said: (Allah says)" And observe prayer for remembrance of Me"