Sahih Bukhari Volume 004, Book 053, Hadith Number 402.

Narated By Abu Huraira : Abu Bakr, on the day of Nahr (i.e. slaughtering of animals for sacrifice), sent me in the company of others to make this announcement: "After this year, no pagan will be allowed to perform the Hajj, and none will be allowed to perform the Tawaf of the Ka'ba undressed." And the day of Al-Hajj-ul-Akbar is the day of Nahr, and it called Al-Akbar because the people call the 'Umra Al-Hajj-ul-Asghar (i.e. the minor Hajj). Abu Bakr threw back the pagans' covenant that year, and therefore, no pagan performed the Hajj in the year of Hajj-ul-Wada' of the Prophets.