Musnad Ahmad 27:714

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, from Ibn 'Umar, that when he was in a hurry to travel, he would combine the Maghrib and Isha prayers after the twilight had disappeared. He would say: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him), when he was in a hurry to travel, would combine them."