Musnad Ahmad 5:43

Sufyan narrated to us, from Ubayd Allah ibn Abi Yazid, from Mujahid, from Ibn Abi Layla, from Ali, that Fatimah came to the Prophet, may Allah bless him and grant him peace, asking him for a servant. He said: "Shall I not guide you to that which is better for you than that? You glorify Allah (say *SubhanAllah*) thirty-three times, you magnify Allah (say *Allahu Akbar*) thirty-three times, and you praise Allah (say *Alhamdulillah*) thirty-three times, making one of them thirty-four."