"كان لا ينام حتى يقرأ {الم، تَنْزِيلُ} السجدة و {تَبَارَكَ الَّذِي بِيَدِهِ الْمُلْكُ}
It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would not sleep without reciting Surah al-Sajdah (Alif Lam Mim, Tanzil) and Surah al-Mulk (Tabarakalladhi biyadihil-mulk)."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4873