Musnad Ahmad Hadith 22110

It is narrated from Mu'adh (may Allah be pleased with him) that he heard the Holy Prophet (peace and blessings of Allah be upon him) say: Whoever receives a wound or suffers pain in the way of Allah, on the Day of Resurrection he will come with it flowing even more, but on that day its color will be like saffron and its fragrance like musk. And whoever receives a wound in the way of Allah, the seal of the martyrs is placed upon him. Whoever sincerely prays to Allah for martyrdom and then departs from this world by natural death, he will receive the reward of a martyr. And any Muslim who fights in the way of Allah even for the time it takes for milk to flow from a she-camel’s teats, Paradise becomes obligatory for him.