Musnad Ahmad 1153:2263

'Abd al-Wahhab ibn 'Ata narrated to us, from Dawud ibn Abi Hind, from al-Sha'bi, from 'Aisha, who said: "Prayer was ordained as two rak'ahs and two rak'ahs, except for the Maghrib, which was ordained as three because it is the witr (odd-numbered prayer)." She said: "And when the Messenger of Allah (peace and blessings of Allah be upon him) traveled, he would pray the first prayer as two rak'ahs, except for the Maghrib. But when he stayed (at a place), he would add two rak'ahs to every two rak'ahs, except for the Maghrib, because it is the witr, and the Subh (dawn prayer), because he would lengthen the recitation in it."