From Sahih Bukhari (Hadith 2972):
Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "Were it not for the fear that it would be difficult for my followers, I would not have remained behind any Sariya, (army-unit) but I don't have riding camels and have no other means of conveyance to carry them on, and it is hard for me that my companions should remain behind me. No doubt I wish I could fight in Allah's Cause and be martyred and come to life again to be martyred and come to life once more
Reference: Bukhari 2972