Musnad Ahmad 962:4

Ruh narrated to us, from Hisham, from Wasil, the freed slave of Abu Uyaynah, from Muhammad ibn Abi Ya’qub, from Raja’ ibn Haywah, from Abu Umamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) initiated a military expedition, so I came to him and said: "O Messenger of Allah, supplicate to Allah for me for martyrdom." He said: "O Allah, keep them safe and grant them spoils." He said: So we were kept safe and we obtained spoils. He said: Then the Messenger of Allah (peace and blessings of Allah be upon him) initiated a second expedition, so I came to him and said: "O Messenger of Allah, supplicate to Allah for me for martyrdom." He said: "O Allah, keep them safe and grant them spoils." He said: So we were kept safe and we obtained spoils. He said: Then he initiated a third expedition, so I came to him and said: "O Messenger of Allah, I came to you twice before this time of mine and asked you to supplicate to Allah for me for martyrdom, but you supplicated to Allah to keep us safe and grant us spoils, and we were kept safe and obtained spoils. O Messenger of Allah, so supplicate to Allah for me for martyrdom." He said: "O Allah, keep them safe and grant them spoils." He said: So we were kept safe and we obtained spoils. Then I came to him and said: "O Messenger of Allah, command me with a deed." He said: "You must fast, for there is nothing like it." He said: So neither Abu Umamah, nor his wife, nor his servant were seen except fasting. He said: If smoke was seen in their house during the day, it was said: "A guest has come to them, or a traveler has descended upon them." He said: I remained like that for as long as Allah willed, then I came to him and said: "O Messenger of Allah, you commanded us to fast, and I hope that Allah has blessed us in it. O Messenger of Allah, so command me with another deed." He said: "Know that you will not prostrate a prostration to Allah except that Allah will raise you a degree by it and remove a sin from you by it."