Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.094B.

There is consensus among the Muslim scholars that spending the Day at Arafah is the most important part of Hajj. Ahmad and the compilers of the Sunan have reported from Abdur Rahman bin Ya’mur that the Prophet (peace be upon him) ordered an announcer to proclaim “Hajj is (spending the day at) Arafah, so he who joins other pilgrims on the night of Muzdalifah before dawn, will be considered as having performed Hajj.”