Hadith: Whoever glorifies Allah directly after each prayer thirty-three times, praises Allah thirty-three times, and proclaims the greatness of Allah thirty-three times, these are ninety-nine, and completes one hundred by saying: La ilāha illallāh wahdahu la sharīka lah, lahu al-mulku wa lahu al-hamdu wa huwa ‘ala kulli shay’in qadīr (There is no god except Allah. He is One and has no partner with Him. To Him belongs sovereignty and to Him belongs praise, and He is Omnipotent over everything), his sins will be forgiven even if they are like the sea foam

عَنْ أَبِي هُرَيْرَةَ رضي الله عنه عَنْ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«مَنْ سَبَّحَ اللهَ فِي دُبُرِ كُلِّ صَلَاةٍ ثَلَاثًا وَثَلَاثِينَ، وَحَمِدَ اللهَ ثَلَاثًا وَثَلَاثِينَ، وَكَبَّرَ اللهَ ثَلَاثًا وَثَلَاثِينَ، فَتْلِكَ تِسْعَةٌ وَتِسْعُونَ، وَقَالَ: تَمَامَ الْمِائَةِ: لَا إِلَهَ إِلَّا اللهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ غُفِرَتْ خَطَايَاهُ وَإِنْ كَانَتْ مِثْلَ زَبَدِ الْبَحْرِ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 597]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Whoever glorifies Allah directly after each prayer thirty-three times, praises Allah thirty-three times, and proclaims the greatness of Allah thirty-three times, these are ninety-nine, and completes one hundred by saying: La ilāha illallāh wahdahu la sharīka lah, lahu al-mulku wa lahu al-hamdu wa huwa ‘ala kulli shay’in qadīr (There is no god except Allah. He is One and has no partner with Him. To Him belongs sovereignty and to Him belongs praise, and He is Omnipotent over everything), his sins will be forgiven even if they are like the sea foam."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 597]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified that whoever says, after completing the obligatory prayer:
Thirty-three times: "Subhānallāh", which is exalting Allah far above deficiencies.
Thirty-three times: "Al-hamdulillāh", which is praising Allah with the attributes of perfection along with loving and glorifying Him.
Thirty-three times: "Allāhu Akbar", which means that Allah is greater and more magnificent than everything.
The completion of the number to one hundred by saying: "La ilāha illallāh wahdahu la sharīka lah, lahu al-mulku wa lahu al-hamdu wa huwa ‘ala kulli shay’in qadīr", which means: No deity is truly worthy of worship except Allah alone with no partner. He alone possesses full sovereignty. He is the only One worthy of praise and commendation along with love and glorification, and He is Omnipotent, and nothing escapes His power.
Whoever says this, his sins will be erased and forgiven, even if they are as abundant as the white foam that appears above the seawater when it is agitated and stormy.