Musnad Ahmad 30:3392

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in Whose hand is the soul of Muhammad, I would love to fight in the way of Allah and be killed, then be brought to life and be killed, then be brought to life and be killed, then be brought to life and be killed. And were it not for the fact that it would be difficult for the believers, I would not remain behind any detachment that goes out—or raids—in the way of Allah. But I do not find the capacity to carry them, and they do not find the capacity to follow me, and their souls would not be content to remain behind me"—or "to sit behind me."