Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 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. "