Sunan Abi Dawud 22469
In-book no 1965حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا حَمَّادٌ، عَنْ أَيُّوبَ، عَنِ الزُّهْرِيِّ، أَنَّ عُثْمَانَ بْنَ عَفَّانَ، أَتَمَّ الصَّلاَةَ بِمِنًى مِنْ أَجْلِ الأَعْرَابِ لأَنَّهُمْ كَثُرُوا عَامَئِذٍ فَصَلَّى بِالنَّاسِ أَرْبَعًا لِيُعْلِمَهُمْ أَنَّ الصَّلاَةَ أَرْبَعٌ .
Narrated Az-Zuhri: Uthman offered complete prayer at Mina for the sake of bedouins who attended (hajj) in large numbers that year. He led the people four rak'ahs in prayer in order to teach them that the prayer (i.e. noon or afternoon prayer) essentially contained four rak'ahs.