Anas bin Malik reported,
The Messenger of Allah ﷺ said that Allah 
says, "The one who strives in the path of Allah is under My guarantee. If I take his soul, I will grant him Paradise as his inheritance, and if I return him, I will return him with reward or spoils of war."He said it is sound but rare from this chain of transmission.
Tirmidhi 1620
Hasanحدثنا محمد بن عبد الله بن بزيع حدثنا المعتمر بن سليمان حدثني مرزوق أبو بكر عن قتادة
عن أنس قال
قال رسول الله ﷺ يعني يقول الله عز وجل المجاهد في سبيل الله هو علي ضامن إن قبضته أورثته الجنة وإن رجعته رجعته بأجر أو غنيمة قال هو صحيح غريب من هذا الوجه
[Hasan] Ibn Abu Asim also narrated it in Kitab al-Jihad, no. 45, from the hadith of Muhammad bin Abdullah bin Bazi, in a shortened form. The hadith has many corroborating narrations in Muslim, no. 1876, and Abu Dawud, no. 2499, 2494, and others.