Sayyiduna Abu Hurayrah (may Allah be pleased with him) says: My dear friend, that is, the Prophet (peace and blessings be upon him), advised me of three things: "Fasting three days of every month, the two rak'ahs of Duha, and that I pray witr before sleeping."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4842