Musnad Ahmad 1461

Muhammad bin ‘AbdurRahman bin "Abdullah bin al Husain narrated from Sa"d bin Abi Waqqas that He would pray Isha" in the mosque of the Messenger of Allah (ﷺ), then he would pray Witr with one rak"ah and no more. He was asked: “Do you pray witr with one rak"ah and no more, O Abu Ishaq?” He said: “Yes, I heard the Messenger of Allah (ﷺ) say: “One who does not sleep until he has prayed Witr is a man of resolve.”