Musnad Ahmad 30:2996

Yahya ibn Sa’id narrated to us, from Yahya—meaning Ibn Sa’id—he said: Dhakwan Abu Salih narrated to me, he said: I heard Abu Hurairah say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that it would be difficult for my Ummah, I would not have remained behind from any expedition that went out. But they do not find mounts, and I do not find what I can mount them upon, and it is difficult for them to remain behind me. I would love to fight in the way of Allah and be killed, then be brought to life, then be killed, then be brought to life, then be killed, then be brought to life."