Sahih Al Bukhari – Book of Prophets

Volumn 004, Book 055, Hadith Number 631.
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Narated By Abdullah bin Amr : Allah's Apostle said to me, "The most beloved fasting to Allah was the fasting of (the Prophet) David who used to fast on alternate days. And the most beloved prayer to Allah was the prayer of David who used to sleep for (the first) half of the night and pray for 1/3 of it and (again) sleep for a sixth of it."