Musnad Ahmad 938:25

'Uthman ibn 'Umar narrated to us, Hisham informed us, from Muhammad, from Kathir ibn Aflah, from Zayd ibn Thabit, who said: We were commanded to glorify Allah (say *SubhanAllah*) thirty-three times after every prayer, to praise Allah (say *Alhamdulillah*) thirty-three times, and to magnify Allah (say *Allahu Akbar*) thirty-four times. Then a man from the Ansar was approached in a dream, and it was said to him: "Did the Messenger of Allah (peace and blessings of Allah be upon him) command you to glorify Allah after every prayer such and such?" The Ansari said in his dream: "Yes." He said: "Then make them twenty-five and twenty-five, and include the declaration of oneness (saying *La ilaha illallah*) in them." When he woke up, he went early to the Prophet (peace and blessings of Allah be upon him) and informed him. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Then do so."