Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that when he was in haste on a journey he would combine Maghrib and Isha after the twilight had disappeared, and he would say that when the Messenger of Allah (peace and blessings be upon him) was in haste on a journey, he would combine Maghrib and Isha.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5513