Musnad Ahmad 834:13

'Abd al-Samad narrated to us, my father narrated to me, Hasan and 'Affan narrated to us, 'Abd al-Warith narrated to us, Husayn narrated to us, 'Abdullah ibn Buraydah narrated to us, from 'Abdullah al-Muzani, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Pray two rak'ahs before the Maghrib prayer." Then he said: "Pray two rak'ahs before the Maghrib prayer." Then he said at the third time: "For whoever wishes," fearing that the people might take it as a Sunnah.