Sahih Muslim Book 4, Hadith Number 1456.

Chapter : The excellence of Qunut in all the prayers when any calamity befalls the Muslims.

Qatada narrated it on the authority of Anas b. Malik that the Apostle of Allah (may peace be upon him) said: He who forgets the prayer, or he slept (and it was omitted), its expiation is this only that he should observe it when he remembers it.