Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

(a Shaikh of the Quraish) "I accompanied Ibn Umar to Al-Hima.* When the sun set I felt too nervous to remind him of the prayer, so he went on until the light on the horizon had disappeared and it was getting dark, then he stopped and prayed Maghrib, three Rakahs, then he prayed two Rakahs immediately afterwards, then he said: 'This is what I saw the Messenger of Allah (saw) do.'" (Sahih) *A place near Madinah.