Musannaf Ibn Abi Shayba 17:1232

Abu Usama narrated to us, from Za'ida, from Mansur, from Ibrahim, who said: Our companions used to command us while we were young boys, when we went to our beds, to glorify Allah (say *SubhanAllah*) thirty times, and praise Allah (say *Alhamdulillah*) thirty-three times, and magnify Allah (say *Allahu Akbar*) thirty-four times.