Sahih Muslim Book 20, Hadith Number 4703.

Chapter : The merit of keeping watch in the way of Allah, the Almighty and Exalted.

It has been narrated on the authority of Salman who said: I heard the Messenger of Allah (may peace be upon him) say: Keeping watch for a day and a night is better (in point of reward) than fasting for a whole month and standing in prayer every night. If a person dies (while, performing this duty), his (meritorious) activity will continue and he will go on receiving his reward for it perpetually and will be saved from the torture of the grave.