The Messenger of Allah ﷺ said, "Whoever glorifies Allah at the end of every prayer thirty-three times, praises Allah thirty-three times, and magnifies Allah thirty-three times, that makes ninety-nine. Then to complete the hundred he says, 'There is no god but Allah alone, with no partner. To Him belongs the dominion and to Him belongs all praise, and He has power over all things,' his sins will be forgiven, even if they were like the foam of the sea."
عن رسول الله ﷺ: من سبح الله في دبر كل صلاة ثلاثا وثلاثين، وحمد الله ثلاثا وثلاثين، وكبر الله ثلاثا وثلاثين. فتلك تسعة وتسعون، وقال: تمام المائة: لا إله إلا الله وحده لا شريك له، له الملك، وله الحمد، وهو على كل شيء قدير، غفرت خطاياه وإن كانت مثل زبد البحر.
Muslim narrated it in the Book of Mosques (597), from Abdul-Hameed bin Bayan Wasiti, who reported to us from Khalid bin Abdullah, from Suhayl, from Abu Ubayd Madhhiji, from Ata bin Yazid Laythi, from Abu Hurairah, and he mentioned it.For more, see Kitab al-Adhkar.