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, to Him belongs all praise, and He is capable of 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 Mosques" (597), from Abdul-Hameed bin Bayan Wasiti, who said, Khalid bin Abdullah informed us, from Suhayl, from Abu Ubayd Madhhaji, from Ata bin Yazid Laythi, from Abu Hurairah, who mentioned it."Dubr" means right after the prayer.