Sayyiduna Ibn Umar (may Allah be pleased with them both) combined Maghrib and Isha at Muzdalifah, and it was said to him: O Abu Abd al-Rahman! What kind of prayer is this? He said: "I combined these two prayers, three rak'ahs of Maghrib and two rak'ahs of Isha, with the Prophet (peace and blessings be upon him) at this place with a single iqamah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9496