Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 359
Narrated Ibn 'Umar:
'Uthman did not join the Badr battle because he was married to one of the daughters of Allah's Apostle and she was ill. So, the Prophet said to him. "You will get a reward and a share (from the war booty) similar to the reward and the share of one who has taken part in the Badr battle."