Sahih al Bukhari 3130
One-fifth of Booty to the Cause of Allah (Khumus)
حَدَّثَنَا مُوسَى، حَدَّثَنَا أَبُو عَوَانَةَ، حَدَّثَنَا عُثْمَانُ بْنُ مَوْهَبٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ إِنَّمَا تَغَيَّبَ عُثْمَانُ عَنْ بَدْرٍ، فَإِنَّهُ كَانَتْ تَحْتَهُ بِنْتُ رَسُولِ اللَّهِ صلى الله عليه وسلم وَكَانَتْ مَرِيضَةً. فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم " إِنَّ لَكَ أَجْرَ رَجُلٍ مِمَّنْ شَهِدَ بَدْرًا وَسَهْمَهُ ".
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 (ﷺ) 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
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.