Musnad Ahmad 27:1046

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, I heard Abu Ishaq narrating that he heard Abdullah ibn Malik al-Hamdani say: I prayed with Ibn Umar at Jam' (Muzdalifah). He performed the iqamah and prayed Maghrib as three rak'ahs. Then he prayed Isha as two rak'ahs with a single iqamah. Khalid ibn Malik asked him about that, and he said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, doing like this in this place."