Sayyiduna Hasan bin Ali (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) taught me: 'Allahumma ihdini fiman hadayt' (O Allah, make me among those whom You have guided), then he mentioned the complete hadith, and at its end it is: "Say these words in the qunut of witr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4628