Rabi' bin Anas says: I was sitting with Sayyiduna Anas (may Allah be pleased with him) when someone asked him: Did the Messenger of Allah (peace and blessings be upon him) recite qunut for only one month? He said: The Messenger of Allah (peace and blessings be upon him) continued to recite qunut in the Fajr prayer until he departed this world.
Abu Abdullah (al-Hakim) said: This chain is sahih (authentic) and its narrators are thiqah (trustworthy). Abu Hatim and Abu Zur'ah described al-Rabi' bin Anas as saduq (truthful) and thiqah (trustworthy). Isma'il bin Muslim al-Makki and Amr bin Ubayd also narrated it from al-Hasan from Sayyiduna Anas (may Allah be pleased with him), but we do not use Isma'il al-Makki or Amr bin Ubayd as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3105