Hadith: The Prophet (may Allah's peace and blessings be upon him) combined the Maghrib and ‘Ishā' prayers (at Muzdalifah) with one Iqāmah. He did not offer any voluntary prayers in between or after either of them

عن عبد الله بن عمر رضي الله عنهما قال: «جَمَع النبيُّ صلى الله عليه وسلم بين المغرب والعشاء بِـجَمْع، لِكُلِّ واحدة منهما إقامة، ولم يُسَبِّحْ بينهما، ولا على إثْرِ واحدةٍ مِنْهُمَا».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) combined the Maghrib and the ‘Ishā' prayers (at Muzdalifah) with one Iqāmah. He did not offer any voluntary prayers in between or after either of them.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

When the sun set on the Day of ‘Arafah, the Prophet (may Allah's peace and blessings be upon him) proceeded to Muzdalifah, where he prayed the Maghrib and the ‘Ishā' prayers, combining them at the time of the ‘Ishā', with one Iqāmah for each prayer, and he did not perform any voluntary prayers between them. he did that in order to have the meaning of combining the prayer fully realized. He did not pray any voluntary prayers after them so that he could take a good portion of rest and be ready for the rituals which he had to perform on the next day.