Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when he was returning from Makkah, news reached him about Sayyidah Safiyyah bint Abi Ubayd (may Allah be pleased with her), so he travelled fast. When the sun set, one of his companions said: The prayer! He remained silent and kept going. Then a companion said again: The prayer! He again remained silent. Then the one who had said "The prayer" said: He knows something about this that I do not know. He kept going, and some time after the twilight had disappeared he dismounted and prayed; on a journey the call to prayer was not given for the prayer either. He combined Maghrib and Isha both, and then said: "When the Prophet (peace and blessings be upon him) was in haste on a journey, the Prophet (peace and blessings be upon him) would combine Maghrib and Isha after the twilight had disappeared." And he would pray voluntary prayers on his mount during the journey, whichever direction the mount faced, and he was informing them that the Prophet (peace and blessings be upon him) used to do the same.
Note: Sayyiduna Ibn Umar (may Allah be pleased with them both) combined these two prayers after the twilight had disappeared; the one who contradicts this is someone who has not memorised the hadiths of Nafi'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5515