Musnad Ahmad 938:84

Rawh narrated to us, Hisham narrated to us, from Muhammad, from Kathir ibn Aflah, from Zayd ibn Thabit, who said: We were commanded to glorify (say *SubhanAllah*) thirty-three times after every prayer, and to praise (say *Alhamdulillah*) thirty-three times, and to magnify (say *Allahu Akbar*) thirty-four times. He said: Then a man saw in a dream, and he said: You were commanded to say thirty-three glorifications, thirty-three praises, and thirty-four magnifications; so if you were to include the declaration of oneness (*Tahlil* - saying *La ilaha illallah*) in them, and make them twenty-five. So I mentioned that to the Prophet, peace and blessings of Allah be upon him, and he said: "You have seen [it], so do [it]," or something similar to that.