Abu Dawud Book 2, Hadith Number 1309.

Chapter : On a person who makes the intention of getting up and praying at night but remains sleeping.

Narated By ‘Aisha, Ummul Mu’minin : The Prophet (PBUH) said: Any person who offers prayer at night regularly but (on a certain night) he is dominated by sleep will be given the reward of praying. His sleep will be almsgiving.