Sunan an-Nasa'i, Volume 3, Book 24, Hadith 3032 (DarusSalam) ⓘ

It was narrated from Ibn Shihab that Ubaidullah bin Abdullah told him that his father said: "The Messenger of Allah joined Maghrib and Isha with no (Voluntary) prayer in between them. He prayed Maghrib with three Rakahs and Isha with two." And Abdullah bin Umar used to join them in like manner until he met Allahm, The Mighty and Sublime.

أَخْبَرَنَا عِيسَى بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا ابْنُ وَهْبٍ، عَنْ يُونُسَ، عَنِ ابْنِ شِهَابٍ، أَنَّ عُبَيْدَ اللَّهِ بْنَ عَبْدِ اللَّهِ، أَخْبَرَهُ أَنَّ أَبَاهُ قَالَ جَمَعَ رَسُولُ اللَّهِ صلى الله عليه وسلم بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ لَيْسَ بَيْنَهُمَا سَجْدَةٌ صَلَّى الْمَغْرِبَ ثَلاَثَ رَكَعَاتٍ وَالْعِشَاءَ رَكْعَتَيْنِ ‏.‏ وَكَانَ عَبْدُ اللَّهِ بْنُ عُمَرَ يَجْمَعُ كَذَلِكَ حَتَّى لَحِقَ بِاللَّهِ عَزَّ وَجَلَّ ‏.‏