Musnad Ahmad 937:223

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, that Abu Kathir, the freed slave of Banu Hashim, narrated to him that he heard Abu Dharr al-Ghifari, the companion of the Messenger of Allah, may Allah bless him and grant him peace, saying: "Words, whoever mentions them one hundred times after every prayer: 'Allah is the Greatest (Allahu Akbar), and Glory be to Allah (SubhanAllah), and Praise be to Allah (Alhamdulillah), and there is no god but Allah alone, He has no partner (La ilaha illallah wahdahu la sharika lah), and there is no power and no strength except by Allah (wa la hawla wa la quwwata illa billah),' then even if his sins were like the foam of the sea, they would be erased." Abu Abd al-Rahman said: My father said: "He did not attribute it to the Prophet (marfu')."