It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) (Malik and Hamzah narrated it as marfu', attributed to the Prophet) that the Messenger of Allah (peace and blessings be upon him) said: "The mu'aqqibat, whose sayer or doer is never disappointed: saying tasbih 33 times, tahmid 33 times, and takbir 34 times after every prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16315