Abu Dawud Book 2, Hadith Number 1292.
Narated By Abdullah Ibn Abbas : The Apostle of Allah (PBUH) said to al-Abbas ibn Abdul Muttalib: Abbas, my uncle, shall I not give you, shall I not present to you, shall I not donate to you, shall I not produce for you ten things? If you act upon them, Allah will forgive you your sins, first and last, old and new, involuntary and voluntary, small and great, secret and open.
These are the ten things: you should pray four rak’ahs, reciting in each one Fatihat al-Kitab and a surah. When you finish the recitation of the first rak’ah you should say fifteen times while standing: “Glory be to Allah”, “Praise be to Allah”, “There is no god but Allah”, “Allah is most great”. Then you should bow and say it ten times while bowing. Then you should raise your head after bowing and say it ten times. Then you should kneel down in prostration and say it ten times while prostrating yourself. Then you should raise your head after prostration and say it ten times. Then you should prostrate yourself and say it ten times. Then you should raise your head after prostrating and say it ten times in every rak’ah. You should do that in four rak’ahs.
If you can observe it once daily, do so; if not, then once weekly; if not, then once a month; if not, then once a year; if not, then once in your lifetime.