Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 92

Narrated Anas:

While the Prophet was in a Ghazwa he said, "Some people have remained behind us in Medina and we never crossed a mountain path or a valley, but they were with us (i.e. sharing the reward with us), as they have been held back by a (legal) excuse. "