Musannaf Ibn Abi Shaybah

Hazrat Ka'b bin Ujrah narrates that the Messenger of Allah, peace and blessings be upon him, said: There are some words that follow after (the prayer), whose reciter is never in loss: After every prayer, say Subhan Allah thirty-three times (33), Alhamdulillah thirty-three times (33), and Allahu Akbar thirty-four times (34).