Musnad Ahmad 1153:2024

Muhammad ibn Abi ‘Adi narrated to us, from Dawud, from al-Sha‘bi, that ‘Aisha said: "The prayer was ordained as two rak‘ahs, two rak‘ahs in Makkah. When the Messenger of Allah (peace and blessings of Allah be upon him) arrived in Madinah, he added two rak‘ahs to every two rak‘ahs, except for Maghrib, for it is the witr (odd-numbered prayer) of the day, and the dawn prayer (Fajr), due to the length of its recitation." He said: "And when he traveled, he would pray the first prayer."