The Prophet (SAS) said "The Day of Adhaa is the day when the people sacrifice their udhiyah."
Al-Imam Al-Khattaabi commented on this hadeeth by saying, "People will be divided into groups and many will go astray from the way of the Prophet (SAS). Some will follow astronomical calculations and some will fast earlier and stand in 'Arafah earlier than the Muslims, like the Baatiniyyah. The Prophet's (SAS) way will always remain and the people who follow it will be the saved ones, those are on the correct path, even if they are few."(Nayl ul-Awtaar 3:329)
Muslims all over the world should follow the hujjaaj in Eid-ul- Adhaa. This is the only opinion that can unite all of the Muslims. The Fuqahaa (RA) (scholars of Fiqh), did not discuss the issue of "ikhtilaaf ulmataali" (the situation where each region has its own moon sighting) or "ittifaaq ulmataali" (the situation where sighting the moon in one place is sufficient for the rest of the world) when it comes to Eid-ul-Adhaa. As a matter of fact, some scholars, such as As-Sarakhsee (from the Hanafee school of thought) in his book 'AlMabsoot' mentioned that there is consensus between the scholars that Muslims should follow the hujjaaj in Eid-ul- Adhaa. 'Arafah is the name of a place, not a day. Whenever Muslims stand in that place during the hajj time, we call that particular day the Day of 'Arafah and the following day is Yawm un-Nahr (Day of Sacrifice) or Eid-ul-Adhaa. Al-Qaadi Zaadah (also from the Hanafee school of thought) said in his treatise, 'Haashiyah', that the day of 'Arafah is the day when Muslims will be standing in 'Arafah." [Haashiyah 2:328] This is why we should follow the hujjaaj in Eid-ul- Adhaa.