Sayyiduna Samurah (may Allah be pleased with him) said: This verse: "This day I have perfected for you your religion, completed My favour upon you, and approved for you Islam as religion" [Surah al-Ma'idah: 3] was revealed on the Day of Arafah, while the Messenger of Allah (peace and blessings be upon him) was standing at Arafat on a Friday.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6886