كتاب الإمارة
33  
The Book on Government
(32)
Chapter: If a person is killed in the cause of Allah, all his sins will be expiated except debt
(32)
باب مَنْ قُتِلَ فِي سَبِيلِ اللَّهِ كُفِّرَتْ خَطَايَاهُ إِلاَّ الدَّيْنَ ‏‏
Sahih Muslim 1886 a

It has been reported on the authority of 'Amr b. al-'As that the Messenger of Allah (ﷺ) said:

All the sins of a Shahid (martyr) are forgiven except debt.
Reference : Sahih Muslim 1886a
In-book reference : Book 33, Hadith 179
USC-MSA web (English) reference : Book 20, Hadith 4649
  (deprecated numbering scheme)