Sahih Bukhari
Times of the Prayers
Volume: 001 / Book Number: 010 / Hadith Number: 571
Narrated Anas:
The Prophet said, "If anyone forgets a prayer he should pray that prayer when he remembers it. There is no expiation except to pray the same." Then he recited: "Establish prayer for My (i.e. Allah's) remembrance." (20.14).