Hadith: Allah has guaranteed for the one who fights in His cause to admit him into Paradise if He takes his soul, or to return him safe and sound with reward or booty.

عن أبي هريرة رضي الله عنه أن النبي صلى الله عليه وسلم قال: «انْتَدَبَ الله (ولمسلم: تَضَمَّنَ الله) لِمَنْ خَرَجَ فِي سَبِيلِهِ، لا يُخْرِجُهُ إلاَّ جِهَادٌ فِي سَبِيلِي، وإيمان بي، وتصديق برسلي فهو عَلَيَّ ضامن: أَنْ أُدْخِلَهُ الجنة، أو أرجعه إلى مسكنه الذي خرج منه، نائلًا ما نال من أجر أو غنيمة». ولمسلم: «مثل المجاهد في سبيل الله -والله أعلم بمن جاهد في سبيله- كَمَثَلِ الصَّائِمِ القائم، وَتَوَكَّلَ الله لِلْمُجَاهِدِ فِي سَبِيلِهِ إنْ تَوَفَّاهُ: أن يدخله الجنة، أو يرجعه سالما مع أجر أو غنيمة».
[صحيح] - [الرواية الأولى: متفق عليها. الرواية الثانية: متفق عليها أيضا]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Allah has assigned (in a wording of Sahih Muslim: Allah guaranteed) for the one who goes out (to fight) in His cause, driven by nothing but (desire for) Jihad in My cause and belief in Me and in My messengers, then he has a guarantee from Me that I will admit him into Paradise or return him to his abode whence he came forth, receiving whatever reward or war booty he managed to obtain." A narration of Muslim reads: "The Mujāhid in the cause of Allah, and Allah knows best who fights in His cause, is like the one who fasts and stands (in prayer at night), and Allah has guaranteed for the one who fights in His cause to admit him into Paradise if He takes his soul, or to return him safe and sound with reward or booty."
[Authentic hadith] - [Narrated by Bukhari & Muslim due to its all chains of narrators]

Explanation

In this Hadīth, there is a guarantee from Allah for the one who leaves in His cause and for whom nothing other than Jihad is the reason for his leaving, as a sincere believer, that he has a promise from Allah for one of three things or two. If he is killed, then he will have a guarantee from Allah to admit him into Paradise, and if he remains alive, then Allah will return him home with any reward and war booty he obtained, meaning: reward without war booty or both. As for the second narration, which was attributed by the author of Al-‘Umdah to Muslim, and which is agreed upon, it states that the virtue of Jihad in the cause of Allah, meaning: what is like Jihad but exceeds the human ability is that, instead of going out in the way of Allah, one enters one's place of prayer and continuously prays and fasts, and this is why the Prophet (may Allah's peace and blessings be upon him) said: "You will not be able to do so."