Narrated Abdullah bin ‘Amr bin Al-‘As:
Allah’s Apostle told me, “The most beloved prayer to Allah is that of David and the most beloved fasts to Allah are those of David. He used to sleep for half of the night and then pray for one third of the night and again sleep for its sixth part and used to fast on alternate days.”
You know why this sort of fast and prayer is most beloved to Allah? Because it’s the hardest way to go about it.