Ubaydullah bin Abdullah bin Umar (may Allah have mercy on him) narrates from his father (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) combined Maghrib and Isha at Muzdalifah, with no prayer between them. He prayed three rak'ahs for Maghrib and two rak'ahs for Isha. Abdullah (may Allah be pleased with them both) used to pray at Muzdalifah in the same way until he met Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5441
Sayyiduna Anas bin Malik (may Allah be pleased with him) says: We set out with the Prophet (peace and blessings be upon him) from Madinah towards Makkah, and the Prophet (peace and blessings be upon him) led us in two rak'ahs for each prayer until we returned to Madinah, except that he led three rak'ahs for Maghrib. The narrator says: We said to Sayyiduna Anas (may Allah be pleased with him): How long did you stay in Makkah? He said: Ten days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5442
Amir al-Sha'bi narrates from Sayyidah Aishah (may Allah be pleased with her) that the prayers were made obligatory as two rak'ahs each, except Maghrib, which was made obligatory as three rak'ahs. When the Prophet (peace and blessings be upon him) travelled, he would pray the original prayer, and when he stayed (as a resident) he would add two rak'ahs to each two, but not in Maghrib, because it is witr (odd), nor in the Fajr prayer, because its recitation is lengthy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5443