Hadith: If anyone says immediately after each prayer "glory be to Allah" thirty-three times, "praise be to Allah" thirty-three times, "Allah is the Greatest" thirty-three times; these are ninety-nine, and completes the hundred with saying "there is no god but Allah. He is One and has no partner with Him. To Him belongs sovereignty and to Him belongs praise, and He is over everything Omnipotent", all his sins will be forgiven for him even if they are as abundant as the foam of the sea

عن أبي هريرة رضي الله عنه عن رسول الله صلى الله عليه وسلم قال: «مَن سبَّح الله دُبُر كل صلاة ثلاثا وثلاثين، وحَمِد الله ثلاثا وثلاثين، وكَبَّر الله ثلاثا وثلاثين، فتِلك تِسْعَةٌ وتِسْعُونَ، وقال تَمَام المائة: لا إله إلا الله وحْدَه لا شريك له، له المُلك، وله الحَمد، وهو على كلِّ شيء قَدِير، غُفِرَت خَطَايَاه، وإن كانت مثل زَبَدِ البَحْرِ».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "If anyone says immediately after each prayer 'glory be to Allah' thirty-three times, 'praise be to Allah' thirty-three times, 'Allah is the greatest' thirty-three times; these are ninety-nine, and completes the hundred with saying 'there is no god but Allah. He is One and has no partner with Him. To Him belongs sovereignty and to Him belongs praise, and He is over everything Omnipotent', all his sins will be forgiven for him even if they are as abundant as the foam of the sea."
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informs us in this Hadīth about the virtues of glorifying, praising, and exalting Allah as well as celebrating His Oneness after the obligatory prayers in particular, as explicitly stated in other narrations. Glorifying Allah means exalting Him above all imperfections and faults. Allah, the Almighty, is entirely perfect in His names, His attributes, and His actions. Praising Allah means praising Him with the attributes of perfection. Glorification implies exalting and freeing Allah from all deficiencies, whereas praise is describing Allah with the attributes of perfection. Exalting the greatness of Allah means describing Him as being greater than anything. To Him belongs the grandeur within the heavens and the earth, and He is the Exalted in Might, the Wise. The virtue mentioned in this Hadīth is forgiveness of sins. It means this formula of Dhikr leads to forgiveness of one’s minor sins and expiation for one’s minor misdeeds. Major sins, however, require repentance in order to be forgiven. Allah, the Almighty,, said: {If you avoid the major sins which you are forbidden, We will remove from you your lesser sins} [Sūrat An-Nisā': 31]. Therefore, anyone says this formula of Dhikr after the obligatory prayers, even if he waits for a short time, whether he is sitting or walking, facing the Qiblah or not, in the mosque or outside it, he will receive the reward in full, for Allah does not do any injustice to anyone. However, if one does not say this Dhikr except after a long time has passed, then he will miss the virtue mentioned above but still receive the reward of making Dhikr in general. It is due to the bounty of Allah, the Almighty, that all man's sins can be forgiven, no matter how numerous they are. This is understood from the Prophet's statement: "his sins will be forgiven even if they are as abundant as the foam of the sea." The number mentioned in the Hadīth requires the Muslim to repeat each part of the formula thirty-three times and say one time "There is no god but Allah. He is One and has no partner with Him. To Him belongs sovereignty and to Him belongs praise, and He is over everything Omnipotent."