Sayyiduna Ammar bin Yasir (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said to me: Offer the witr before you sleep, and the (voluntary) night prayer is offered two by two.
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains al-Rabi' bin Badr, who is da'if (weak).