Hadith: He did a little but was greatly rewarded

عن البراء رضي الله عنه قال: أتَى النبي صلى الله عليه وسلم رَجُلٌ مُقَنَّعٌ بالحَديد، فقال: يا رسول الله، أُقَاتِلُ أَوْ أُسْلِمُ؟ قال: «أَسْلِم، ثم قَاتل»، فأسْلَم ثم قاتل فَقُتِل. فقال رسول الله صلى الله عليه وسلم : «عَمِل قليلاً وأُجر كثيراً».
[صحيح] - [متفق عليه]

Al-Barā' (may Allah be pleased with him) reported: A man wearing an iron armor came to the Prophet (may Allah's peace and blessings be upon him) and said: "O Messenger of Allah, shall I fight or embrace Islam?" He said: "Embrace Islam, then fight." He embraced Islam, then fought and was killed. The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "He did a little but was greatly rewarded."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

A man came to the Prophet (may Allah's peace and blessings be upon him) wishing to fight in the cause of Allah with him. He was completely covered in an iron armor, but had not yet entered Islam. He said: "O Messenger of Allah, do I fight in the cause of Allah and then enter Islam, or enter Islam then fight in the cause of Allah?" he said to him: "Embrace Islam, then fight in the cause of Allah." So, the man embraced Islam, then fought in the cause of Allah until he was killed. Thereupon, the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "He did a little but was greatly rewarded", i.e. he did a little and was rewarded a lot in relation to the period he lived as a Muslim, as the time between the man's embracing of Islam and his death was a short one, yet he was rewarded in abundance. That is because fighting in the cause of Allah, Exalted, in order to raise high His Word is one of the most virtuous deeds and most tremendous in reward.