It was narrated from Abu Sa’eed Al-Khudri that the Prophet (ﷺ) said: “The one who fights in the cause of Allah has a guarantee from Allah. Either He will raise him to His forgiveness and mercy, or He will send him back with reward and spoils of war. The likeness of the one who fights in the cause of Allah is that of one who fasts and prays at night without ceasing, until he returns.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، عَنْ شَيْبَانَ، عَنْ فِرَاسٍ، عَنْ عَطِيَّةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " الْمُجَاهِدُ فِي سَبِيلِ اللَّهِ مَضْمُونٌ عَلَى اللَّهِ إِمَّا أَنْ يَكْفِتَهُ إِلَى مَغْفِرَتِهِ وَرَحْمَتِهِ وَإِمَّا أَنْ يَرْجِعَهُ بِأَجْرٍ وَغَنِيمَةٍ وَمَثَلُ الْمُجَاهِدِ فِي سَبِيلِ اللَّهِ كَمَثَلِ الصَّائِمِ الْقَائِمِ الَّذِي لاَ يَفْتُرُ حَتَّى يَرْجِعَ " .
sunnah.com