بُلُوغُ الْمَرَامِ
Bulugh al-Maram 188
The Book of Prayer
وَلَهُ عَنْ اِبْنِ عُمَرَ: { جَمَعَ بَيْنَ اَلْمَغْرِبِ وَالْعِشَاءِ بِإِقَامَةٍ وَاحِدَةٍ } [1] .
Muslim also reported from Ibn 'Umar (RA): The Prophet (ﷺ) combined the Maghrib and 'Isha prayers with one Iqamah.
Sunnah.com reference: Book 2, Hadith 48