Musnad Ahmad 962:84

Yazid narrated to us, Mahdi ibn Maymun informed us, from Muhammad ibn Abi Ya’qub, from Raja’ ibn Haywah, from Abu Umamah, who said: The Messenger of Allah (ﷺ) 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." So we went on the expedition, and we were kept safe and granted spoils. Then he initiated another military expedition, and 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." So we went on the expedition, and we were kept safe and granted spoils. Then he initiated another military expedition, and I came to him and said: "O Messenger of Allah, I have come to you successively three times, asking you to supplicate to Allah for me for martyrdom, and you said: 'O Allah, keep them safe and grant them spoils.' So we went on the expedition, and we were kept safe and granted spoils. So command me, O Messenger of Allah, with a matter by which Allah may benefit me." He said: "You must fast, for there is nothing like it." It was said: Abu Umamah would hardly ever be seen with smoke in his house during the day. If smoke was seen during the day, they knew that a guest had come upon them, because of how much he and his family would fast. He said: So I came to the Prophet (ﷺ) and said: "O Messenger of Allah, you commanded me with a matter that I hope Allah has benefited me with, so command me with another matter." He said: "Know that you do not prostrate a prostration to Allah except that Allah raises you a degree by it and removes a sin from you by it."