Hadith: There are certain things, the sayer of which – or the performer of which – after every prescribed prayer will never be disappointed: Tasbīh (saying 'Subhānallāh') thirty-three times, Tahmīd (saying 'Al-hamdulillāh') thirty-three times, and Takbīr (saying 'Allāhu Akbar') thirty-four times

عَنْ كَعْبِ بْنِ عُجْرَةَ رضي الله عنه عَنْ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«مُعَقِّبَاتٌ لَا يَخِيبُ قَائِلُهُنَّ -أَوْ فَاعِلُهُنَّ- دُبُرَ كُلِّ صَلَاةٍ مَكْتُوبَةٍ، ثَلَاثٌ وَثَلَاثُونَ تَسْبِيحَةً، وَثَلَاثٌ وَثَلَاثُونَ تَحْمِيدَةً، وَأَرْبَعٌ وَثَلَاثُونَ تَكْبِيرَةً».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 596]

Ka‘b ibn ‘Ujrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"There are certain things, the sayer of which – or the performer of which – after every prescribed prayer will never be disappointed: Tasbīh (saying 'Subhānallāh') thirty-three times, Tahmīd (saying 'Al-hamdulillāh') thirty-three times, and Takbīr (saying 'Allāhu Akbar') thirty-four times."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 596]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed about certain Adhkār (regular supplications) whose sayer will never lose or regret; rather, he will attain a reward for saying such words that succeed each other and that are to be said after the obligatory prayer. These Adhkār are:
"Subhānallāh" (glory be to Allah) thirty-three times, i.e., exalting Allah Almighty above any deficiency.
"Al-hamdulillāh" (praise be to Allah) thirty-three times, i.e., describing Allah with complete perfection, along with loving and extolling Him.
"Allāhu Akbar" (Allah is the Greatest) thirty-three times, as Allah is greater, more magnificent, and mightier than everything.