Sunnah.com reference : Book 2, Hadith 217English translation : Book 2, Hadith 322Arabic reference : Book 2, Hadith 324
Narrated Abu Hurairah (RA) Allah's Messenger (ﷺ) said:
"Whoever glorifies Allah (by saying Subhan-Allah) after every Salat (prayer) thirty-three times, and praises Allah (by saying Alhamdu-lillah) thirty-three times, and exalts Allah (by saying Allahu Akbar) thirty-three times, those are ninety-nine in all, and says to complete a hundred: La ilaha ill-Allahu, wahdahu la sharika lahu, lahul-mulku wa lahulhamdu, wa Huwa 'ala kulli shai'in Qadir (There is nothing which deserves to be worshipped except Allah Alone Who has no partner; to Him belongs the kingdom, to Him praise is due, and He has power over everything), his sins will be forgiven, even if they are as abundant as the foam of the sea."

[Reported by Muslim].
وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- عَنْ رَسُولِ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ : { مَنْ سَبَّحَ اَللَّهَ دُبُرَ كُلِّ صَلَاةٍ ثَلَاثًا وَثَلَاثِينَ , وَحَمِدَ اَللَّهِ ثَلَاثًا وَثَلَاثِينَ , وَكَبَّرَ اَللَّهُ ثَلَاثًا وَثَلَاثِينَ , فَتِلْكَ تِسْعٌ وَتِسْعُونَ , وَقَالَ تَمَامَ اَلْمِائَةِ : لَا إِلَهَ إِلَّا اَللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ , لَهُ اَلْمُلْكُ , وَلَهُ اَلْحَمْدُ , وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ , غُفِرَتْ لَهُ خَطَايَاهُ , وَإِنْ كَانَتْ مِثْلَ زَبَدِ اَلْبَحْرِ }  رَوَاهُ مُسْلِمٌ .‏  1‏‏1 ‏- صحيح .‏ رواه مسلم ( 597 )‏ .‏