Husayn bin al-Harith al-Jadali says that the governor of Makkah delivered a sermon and said: "The Messenger of Allah (peace and blessings be upon him) charged us to perform the sacrifice upon sighting it. If we do not see it and two upright witnesses testify, we perform the sacrifice on their testimony." I asked Husayn bin al-Harith: Who is the governor of Makkah? He said: I do not know. Then he met me after that and said: He is al-Harith bin Hatib, the brother of Muhammad bin Hatib. Then the governor said: "Among you is one who knows Allah and His Messenger (peace and blessings be upon him) better than I, and he witnessed this from the Messenger of Allah (peace and blessings be upon him)," and he pointed with his hand towards a man. Husayn says: I said to an elderly man beside me: Who is this to whom the governor pointed? He said: This is Abdullah bin Umar (may Allah be pleased with them both), and he spoke the truth; he was indeed more knowledgeable than him. Then he (Ibn Umar) said: "This is what the Messenger of Allah (peace and blessings be upon him) commanded us."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8185