Raja' bin Haywah (may Allah have mercy on him) narrates from Sayyiduna Abu Umamah (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) sent out an army, and I went out with them. I said: O Messenger of Allah, supplicate to Allah that He grant me martyrdom. He (peace and blessings be upon him) said: "O Allah, keep them safe and grant them spoils." We returned safely and also gained spoils. Then he (peace and blessings be upon him) sent out another army, and I went out with it too and said: O Messenger of Allah, supplicate to Allah that He grant me martyrdom. He (peace and blessings be upon him) said: "O Allah, keep them safe and grant them spoils." Then the third time the same happened, so I said: O Messenger of Allah, I have come to you three times asking you to supplicate for martyrdom for me, and each time you said: "O Allah, keep them safe and grant them spoils." We have returned safely and gained spoils, O Messenger of Allah, so now tell me of a deed. He (peace and blessings be upon him) said: "Hold fast to fasting, for there is no deed like it and none equal to it." Abu Umamah (may Allah be pleased with him) said: I was granted good through it.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7433