Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) sent Sayyiduna Abu Bakr (may Allah be pleased with him) as the leader of the Hajj and ordered him to announce these matters. Sayyiduna Abu Bakr (may Allah be pleased with him) halted at some place on the way and heard the voice of al-Qaswa', the she-camel of the Messenger of Allah (peace and blessings be upon him). He came out alarmed, but it was Sayyiduna Ali (may Allah be pleased with him). He gave Sayyiduna Abu Bakr (may Allah be pleased with him) the letter of the Prophet (peace and blessings be upon him), in which it was stated that Sayyiduna Ali (may Allah be pleased with him) should be allowed to announce these matters. When Sayyiduna Abu Bakr and Sayyiduna Ali (may Allah be pleased with them both) performed Hajj, Sayyiduna Ali (may Allah be pleased with him) announced these matters during the days of Tashriq: that Allah and the Messenger are free of every polytheist. "So travel freely in the land for four months, and know that you cannot frustrate Allah" [Surah al-Tawbah: 2]. 1: "After this year no polytheist shall perform Hajj"; 2: "The House of Allah shall not be circumambulated with clothes taken off"; 3: "None will enter Paradise except a believer." Sayyiduna Abu Hurayrah (may Allah be pleased with him) used to relay his voice onward.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18820