Sunan Abi Dawud Book 15, Hadith 23
AbuMalik heard the Messenger of Allah (ﷺ) say: He who goes forth in Allah's path and dies or is killed is a martyr, or has his neck broken through being thrown by his horse or by his camel, or is stung by a poisonous creature, or dies on his bed by any kind of death Allah wishes is a martyr and will go to Paradise.
The Messenger of Allah (ﷺ) said: (The reward of) prayer, fasting and remembrance of Allah is enhanced seven hundred times over (the reward of) spending in Allah's path.
The Prophet (ﷺ) said: Everyone who dies will have fully complete his action, except one who is on the frontier (in Allah's path), for his deeds will be made to go on increasing till the Day of Resurrection, and he will be safe from the trial in the grave.