Musnad Ahmad 27:1057

Muhammad narrated to us, Shu'bah narrated to us, from Salamah ibn Kuhayl, who said: I witnessed Sa'id ibn Jubayr at Jam' (Muzdalifah), and he established the prayer. He prayed Maghrib as three [rak'ahs] and performed the taslim, and he prayed the 'Atamah (Isha) as two rak'ahs. Sa'id narrated that Abdullah ibn Umar prayed it in this place and did the same as this. And Ibn Umar narrated that the Messenger of Allah (peace and blessings of Allah be upon him) did the same as this in this place.