Al-Bara (May Allah be pleased with him) reported:
A man equipped with arms came to the Prophet ﷺ and asked: "O Messenger of Allah! Should I go and fight or should I embrace Islam first?" He ﷺ replied, "Enter in the fold of Islam and then fight." He embraced Islam and fought until he was killed. Thereupon the Messenger of Allah ﷺ said, "He accepted Islam for a short time but was rewarded much."

[Al-Bukhari and Muslim].

وعن البراء رضي الله عنه قال أتى النبي ﷺ رجل مقنع بالحديد، فقال يا رسول الله أقاتل أو أسلم؟ قال: "أسلم، ثم قاتل" فأسلم، ثم قاتل فقتل، فقال رسول الله ﷺ : عمل قليلا وأُجر كثيرًا" ((متفق عليه وهذا لفظ البخاري)) .