Musnad Ahmad 27:1795

Mubashir ibn Isma'il narrated to us, Al-Awza'i narrated to us, from Al-Zuhri, from Salim, from his father, who said: "I prayed the Isha prayer with the Messenger of Allah (ﷺ) at Mina, two rak'ahs; and with Abu Bakr, two rak'ahs; and with Umar, two rak'ahs; and with Uthman, two rak'ahs, during the beginning of his caliphate, then he completed it (four rak'ahs) after that."