Musnad Ahmad 30:3137

Surayj narrated to us, he said: Fulayh narrated to us, from Suhayl—meaning Ibn Abi Salih—from Abu Ubayd, from Ata’ ibn Yazid, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever glorifies Allah (says SubhanAllah) thirty-three times, and magnifies Allah (says Allahu Akbar) thirty-three times, and praises Allah (says Alhamdulillah) thirty-three times, and says: '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 illa Allah wahdahu la sharika lah, lahu al-mulk wa lahu al-hamd, wa huwa 'ala kulli shay'in qadir) after the prayer, his sins will be forgiven, even if they were more than the foam of the sea."