Sayyiduna Samurah (may Allah be pleased with him) narrates: the Prophet (peace and blessings be upon him) commanded us to pray (voluntary) prayer at night, whether little or much, and to make the last of it Witr.
Al-Bazzar, al-Tabarani in al-Mu'jam al-Awsat and al-Mu'jam al-Kabir, and Abu Ya'la recorded it; al-Bazzar also has, in one narration, these words: "the Prophet (peace and blessings be upon him) would command us to pray (voluntary) prayer every night after the obligatory prayer," followed by the hadith as before; and its chain is weak.