Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.095.

Section : Time for Arafah.

Most Muslim scholars are of the opinion that the time to spend in Arafah begins from noon of the 9th day of Zhul-Hijjah until dawn of the 10th of Zhul-Hijjah, and that any part of this period of time, day or night, may be spent in Arafah (to fulfil this condition). A pilgrim choosing to spend the day time at Arafah must stay there until after the sunset. But if someone spends the night at Arafah he is not obligated to stay for any specific length of time. Ash-Shafi’i holds that extending the stay at Arafah until nightfall is sunnah.