On the 9th of Dhu al-Hijjah, 10 AH, the Prophet ﷺ stood in the Valley of Aranah at Arafah and addressed the huge crowd of pilgrims. That year the day fell on a Friday, and he called it the Day of al-Hajj al-Akbar (the Great Hajj). He asked his uncle Abbas RA and his cousin Ali RA to quieten the crowd so that his words could carry. Some sahaba reported that they could hear him even from their tents, which is counted among his miracles.
It was on this occasion that Allah revealed the verse, "This day, I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion" [Qur'an, 5:3]. It was one of the last verses to be revealed, though not the very last.
The Prophet ﷺ opened by saying, "O people, listen to me, for I know not whether I shall meet you again after this year." He declared that the blood, wealth and honour of every Muslim were sacred, as sacred as that day, that month and that land. He abolished the customs of the days before Islam (Jahiliyyah), and he began with his own family. First he cancelled the unpaid blood money his family was owed by the tribe of Hudhayl. Then he cancelled the riba (interest) still owed to his uncle al-Abbas RA. He told the people to fear Allah in how they treated women, and he set out the rights that husbands and wives have over each other.
He told them that he had left them the Book of Allah, and that as long as they held to it they would never go astray. He then told them that they would soon be asked about him, and asked what they would say. The people replied that they would testify he had conveyed the message, done his duty and been sincere with them. He raised his hands and said three times, "O Allah, bear witness."