Abu Dawud Book 7, Hadith Number 2331.
Narated By Abdullah ibn Umar : Husayn ibn al-Harith al-Jadli from the tribe of Jadilah Qays said: The governor of Mecca delivered a speech and said: The Apostle of Allah (PBUH) took a pledge from us that we should perform the rites of hajj after sighting the moon. If we do not sight it and two reliable persons bear witness, we should perform the rites of hajj on the basis of their witness.
I then asked al-Husayn ibn al-Harith: Who was the governor of Mecca? He replied: I do not know. He then met me later on and told me: He was al-Harith ibn Hatib, brother of Muhammad ibn Hatib. The governor then said: There is among you a man who is more acquainted with Allah and His Apostle than I. He witnessed this from the Apostle of Allah (PBUH). He then pointed with his hand to a man. Al-Husayn said: I asked an old man beside me: Who is that man to whom the governor has alluded?
He said: “This is Abdullah ibn Umar, and he spoke the truth. He was more acquainted with Allah than he. He (Abdullah ibn Umar) said: For this is what the Apostle of Allah (PBUH) commanded us (to do).