Musnad Ahmad 27:25

Ishaq ibn Yusuf al-Azraq narrated to us, from Ubayd Allah—meaning Ibn Umar—from Nafi, from Ibn Umar: that he used to combine the two prayers, the Maghrib and the Isha, when the twilight (ash-shafaq) had disappeared. He said: And the Messenger of Allah (peace and blessings of Allah be upon him) used to combine them when the journey became urgent for him.