Sahih Bukhari Volume 4, Book 52, Hadith Number 216.

Narated By Abu Huraira : Allah’s Apostle 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.”