It is narrated from Sayyiduna Urwah bin Mudarris bin Aws bin Harithah (may Allah be pleased with him) that he performed Hajj in the time of the Messenger of Allah (peace and blessings be upon him). He could only reach the people at night, while they were at Muzdalifah. So he went to Arafat at night and departed from there, then returned to Muzdalifah, came to the Messenger of Allah (peace and blessings be upon him) and said: "I have exhausted myself and wearied my mount; so has my Hajj been accomplished?" He (peace and blessings be upon him) said: "Whoever prays the Fajr prayer with us at Muzdalifah and stands with us until we depart, having already departed from Arafat before that, by night or by day, his Hajj is complete and he has removed his grime (completed his rites)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14449