He was called to attend to one of his family members, so he traveled hastily and delayed the Maghrib prayer until the twilight had disappeared. Then he dismounted and combined the two prayers. He then informed them that the Messenger of Allah ﷺ used to do this whenever he was traveling in haste.
حدثنا هناد بن السري حدثنا عبدة بن سليمان عن عبيد الله بن عمر عن نافع
أنه استغيث على بعض أهله فجد به السير فأخر المغرب حتى غاب الشفق ثم نزل فجمع بينهما ثم أخبرهم أن رسول الله ﷺ كان يفعل ذلك إذا جد به السير
No references are available.
Muslim also narrated it in the book of the prayer of travelers, chapter on the permissibility of combining two prayers, hadith no. 703, from the hadith of Ubaydullah bin Umar, with the same chain.