Musnad Ahmad 30:1713

Muhammad ibn al-Sabah narrated to us, he said: Isma'il ibn Zakariyya narrated to us, from Suhayl ibn Abi Salih, from Abu 'Ubayd, from 'Ata' ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever glorifies Allah (says SubhanAllah) at the end of every prayer thirty-three times, and praises Allah (says Alhamdulillah) thirty-three times, and magnifies Allah (says Allahu Akbar) thirty-three times, that is ninety-nine." Then he said: "To complete the hundred: 'There is no god but Allah alone, He has no partner, to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things' (La ilaha illallah wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir). His sins will be forgiven, even if they were like the foam of the sea."