Sahih Muslim 1913 a
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.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَبْدِ الرَّحْمَنِ بْنِ بَهْرَامَ الدَّارِمِيُّ، حَدَّثَنَا أَبُو الْوَلِيدِ الطَّيَالِسِيُّ،
 حَدَّثَنَا لَيْثٌ، - يَعْنِي ابْنَ سَعْدٍ - عَنْ أَيُّوبَ بْنِ مُوسَى، عَنْ مَكْحُولٍ، عَنْ شُرَحْبِيلَ بْنِ السَّمِطِ،
 عَنْ سَلْمَانَ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏
"‏ رِبَاطُ يَوْمٍ وَلَيْلَةٍ خَيْرٌ مِنْ
 صِيَامِ شَهْرٍ وَقِيَامِهِ وَإِنْ مَاتَ جَرَى عَلَيْهِ عَمَلُهُ الَّذِي كَانَ يَعْمَلُهُ وَأُجْرِيَ عَلَيْهِ رِزْقُهُ وَأَمِنَ
 الْفَتَّانَ ‏"‏ ‏.‏