Abu Kathir, the freed slave of Banu Hashim, narrates that he heard Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him), the Companion of the Prophet (peace and blessings be upon him), say: There are some words which are recited 100 times after every prayer: 'Allahu akbar', 'Subhan Allah', 'Al-hamdu lillah' and this phrase: 'La ilaha illallahu wahdahu la sharika lah, wa la hawla wa la quwwata illa billah' (There is no god except Allah Almighty, He alone is the only one worthy of worship, He has no partner, and nothing can happen except with the help of Allah Almighty). If that person's sins were as much as the foam of the sea, these words would erase them.
Ahmad recorded it as a mawquf narration (stopping at the Companion); I do not know the narrator named Abu Kathir, and the hadith narrated by the rest of its narrators is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأذكار / 16916