Musnad Ahmad 32:537

Hassan narrated to us, Ibn Lahi'ah narrated to us, from Bukayr ibn al-Ashajj, from Muhammad ibn Abdullah ibn Abi Sulaym, from Anas ibn Malik, who said: "The Messenger of Allah (ﷺ) prayed the prayer in Mina as two rak'ahs. Abu Bakr prayed it in Mina as two rak'ahs, and Umar prayed it in Mina as two rak'ahs, and Uthman ibn Affan prayed it in Mina as two rak'ahs for four years, then he completed it [as four rak'ahs] after that."