Musnad Ahmad 27:446

'Abd al-Razzaq narrated to us, Sufyan informed us, from Salamah ibn Kuhayl, from Sa'id, from Ibn 'Umar, and from Abu Ishaq, from 'Abdullah ibn Malik al-Asadi, from Ibn 'Umar: "That the Prophet, peace and blessings of Allah be upon him, combined the Maghrib and Isha prayers at Jam' (Muzdalifah). He prayed Maghrib as three [rak'ahs] and Isha as two [rak'ahs], with a single iqamah."