Sunan ad-Darimi

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that Sayyiduna Abu Dharr (may Allah be pleased with him) said: O Messenger of Allah! The wealthy people have taken all the rewards; they pray as we pray, and they fast as we fast, but they give charity from their surplus wealth, and we have nothing to give in charity. The Messenger of Allah (peace and blessings be upon him) said: “Shall I not tell you of an action, that if you adhere to it, you will catch up with those who have surpassed you (in reward), and no one will reach your rank except one who does the same as you?” Sayyiduna Abu Dharr (may Allah be pleased with him) said: O Messenger of Allah! Do tell me of such an action. So he (peace and blessings be upon him) said (that action is): After every prayer, say «سبحان الله» thirty-three times, and say «الحمد لله» thirty-three times, and say «الله اكبر» thirty-three times, and complete one hundred with «لَا إِلٰهَ إِلَّا اللّٰهُ وَحْدَهُ لَا شَرِيْكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَىٰ كُلِّ شَيْيءٍ قَدِيْر» at the end.

Sayyiduna Zaid bin Thabit (may Allah be pleased with him) said: We were commanded to say «سبحان الله» 33 times, «الحمد لله» 33 times, and «الله اكبر» 34 times after every prayer. Then a man from among the Ansar was told in a dream: Has your Prophet commanded you to say tasbih 33 times, tahmid 33 times, and takbir 34 times after every prayer? He said: Yes. So he (the one in the dream) said: Say them 25 times each, and say «لا اله الا الله» 25 times. When this dream was mentioned to the Prophet Muhammad (peace and blessings be upon him), he said: "Do so."