Musnad Abu Ya'la al-Mawsili 154:162

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Sufyan bin Sa'id informed us, from Abu Ishaq, from Abdullah bin Malik, who said: I prayed Maghrib and Isha with Ibn Umar at Muzdalifah. He prayed Maghrib as three [rak'ahs] and Isha as two rak'ahs, with an iqamah. When he finished, a man said to him, "What is this prayer, O Abu Abd al-Rahman?" He said, "I prayed it with the Messenger of Allah, may Allah bless him and grant him peace."