Arabic text
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».
Translation
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."
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.
Source: HadeethEnc.com