From Sahih Muslim (Hadith 4938):
It has been narrated on the authority of Salman who said:I heard the Messenger of Allah (ﷺ) 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
Reference: Muslim 4938