Musnad Ahmad 1076:1

Mu'tamir narrated to us, from his father, from a man, from 'Ubayd, the freed slave of the Prophet (peace and blessings of Allah be upon him), who said: He was asked: "Did the Messenger of Allah (peace and blessings of Allah be upon him) use to command prayer after the obligatory one, or other than the obligatory one?" He said: "Yes, between the Maghrib and the 'Isha'."