Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever after every prayer recites 'Subhan Allah' 33 times, 'Al-hamdu lillah' 33 times and 'Allahu akbar' 33 times, that makes ninety-nine; and to complete the hundred recites once 'La ilaha illallahu wahdahu la sharika lahu, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir' (There is no god but Allah, He is alone, He has no partner; to Him belong the dominion and the praise, and He has power over all things), his sins will be forgiven even if they are like the foam of the sea."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3025