Sunan Abi Dawud — The Rites of Hajj (Kitab Al-Manasik Wa'l-Hajj) — Hadith No: 1964
Translation
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