Musnad Ahmad 27:445

'Abd al-Razzaq narrated to us, Sufyan informed us, from Abu Ishaq, from 'Abdullah ibn Malik, from Ibn 'Umar, who said: I prayed Maghrib with him [the Prophet] as three [rak'ahs], and Isha as two rak'ahs, with a single iqamah. Malik ibn Khalid al-Harithi 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, in this place with a single iqamah."