Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates: The Prophet (peace and blessings be upon him) said: "Three things are obligatory upon me, and they are Sunnah for you: Witr, using the tooth-stick (miswak), and offering voluntary prayers at night."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Musa bin Abd al-Rahman al-San'ani, who is kadhdhab (a liar).