Sunan an Nasai
3168
Sahih+2

It was narrated that Salman said:"I heard the Messenger of Allah (ﷺ) say: 'Whoever guards Ribat (the frontier) in the cause of Allah for one day and one night, he will have (a reward) like that of fasting and praying Qiyam for a month. If he dies he will continue to receive reward for what he did, and he will be kept safe from Al-Fattan, and he will be given provision

SahihAbu Ghuddah
SahihAl-Albani