Musnad Ahmad 8:23

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn 'Abd al-Rahman ibn 'Abd Allah ibn al-Husayn narrated to me, that he narrated from Sa'd ibn Abi Waqqas, that he used to pray the last Isha prayer in the mosque of the Messenger of Allah, may Allah bless him and grant him peace, then he would perform Witr with one [rak'ah], not adding to it. He said: It was said to him, "Do you perform Witr with one [rak'ah] and not add to it, O Abu Ishaq?" He said: "Yes, for I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'He who does not sleep until he performs Witr is prudent.'"