It is narrated from Sayyiduna Abu Malik al-Ash'ari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The Promised Day is the Day of Resurrection; the witnessing day is Friday, and the day that is witnessed is the Day of Arafah. Friday is a day that Allah has reserved for us, and the middle prayer is the Asr prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3445