What is the evidence that it is obligatory to stay overnight in Muzdalifah?
Question

                What is the evidence that it is obligatory to stay overnight in Muzdalifah?.
Praise be to Allah.
    The evidence that it is obligatory is the verse in which 
    Allah says (interpretation of the meaning): “There is no sin on you if 
    you seek the Bounty of your Lord (during pilgrimage by trading, etc.). Then 
    when you leave Arafat, remember Allah (by glorifying His Praises, i.e. 
    prayers and invocations, etc.) at the Mashar-il-Haram. And remember Him (by 
    invoking Allah for all good, etc.) as He has guided you, and verily, you 
    were, before, of those who were astray” [al-Baqarah 2:198]. 

    The basic principle with regard to commands is that they are 
    obligatory unless there is evidence to indicate otherwise. And because the 
    Prophet (blessings and peace of Allah be upon him) said to ‘Urwah ibn 
    Mudarris (may Allah be pleased with him), when he met him at Fajr prayer on 
    the day of Muzdalifah, when he said: O Messenger of Allah, I have worn 
    myself out and I have exhausted my camel; there is no hill on which I did 
    not stand. The Messenger of Allah (blessings and peace of Allah be upon him) 
    said: “Whoever attended this prayer of ours and stood with us until we moved 
    on, and stood in ‘Arafah before that, by night or day, has completed his 
    Hajj and completed his rituals.”

     And because the Prophet (blessings and peace of Allah be 
    upon him) granted a concession to the weak allowing them to move on from 
    Muzdalifah at the end of the night, and the concession indicates that the 
    original principle is that it was a duty and a must. In fact some of the 
    scholars are of the view that stopping in Muzdalifah is one of the pillars 
    or essential parts of Hajj, because Allah, may He be exalted, enjoined it in 
    the verse in which He said (interpretation of the meaning):

    “Whoever attended this prayer of ours and stood with us 
    until we moved on, and stood in ‘Arafah before that, by night or day, has 
    completed his Hajj and completed his rituals”

    [al-Baqarah 2:198]. 

    And the Prophet (blessings and peace of Allah be upon him) 
    adhered to that and said: “I have stopped here, and all of Jam‘ – i.e. 
    Muzdalifah – is a place of stopping.” Narrated by Muslim. But the moderate 
    scholarly view is that staying overnight in Muzdalifah is obligatory but is 
    not an essential part or pillar and is not a Sunnah. End quote. 

    Majmoo‘ Fataawa Ibn ‘Uthaymeen, 23/52.