Musannaf Ibn Abi Shayba 3:2143

Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, from 'Urwah ibn al-Zubayr, that he once prayed the Maghrib prayer as two rak'ahs, then he performed the taslim. His leader (or attendant) spoke to him, and his leader said to him, "You only prayed two rak'ahs." So he prayed a rak'ah, then performed the taslim, and prostrated two prostrations (of forgetfulness). Then he said, "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) did the same as this."