A flat open area between mina and Arafat, in the province of Makkah, is name as Muzdalifah. It is one of the four important place related to Hajj. At one end it has Valley of Muhassar and at the other end lays the Mountains of Ma’zamayn.
On 9th of Zul Hajj pilgrims leave for Arafat from Mina, where they spend their whole day till asar prayer. Then they travel back towards Muzdalifah without offering Maghrib prayer.
When they reach Muzdalifah at night then both Maghrib and Isha’a prayers are offered combined after a single Adhan and with two Aqamats; three rakahs of Maghrib and four rakahs of Isha’a – no sunnah and witr. For this journey it is masnoon (advised) not to run or cross over others; but to walk on a steady pace and while reciting talbih.
The night is spent in open air and until Fajr/Subh Sadiq and sunrise the Hujjaj have to leave for Mina; for pelting of Jamarah with the pebbles collected in Muzdalifah through the night. The old, sick and people with kids are allowed to leave muzdalifah even before fajr so as to reach mina on time; this is to ease the difficulty of journey for women, kids and elder people.
Also it is allowed to pray fajr prayer soon after aadhan and not to wait for light to appear, so that one can leave for mina sooner.
Both of the above permissions are mentioned in Bukhari and Muslim:
There is a hill here named Mash’arul Haram; a open roof masjid is also built on this hill. It is masnoon (advised) to offer dua’a near this Masjid. This importance of this Hill is also mentioned in Qura’an Surah Baqarah “When you leave Arafat, then remember Allah at the Mash’arul Haram.” In this ayah Muzdalifah is referred with the name of the hill.
Waqoof (stay) Muzdalifah is wajib (mandatory) even not whole night only a part of it. According to one hadith narrated by Arwa (RA), anyone who is able to offer only Fajr Salah in Muzdalifah, has accomplished his Waqoof-e-Muzdalifah.
At the end of Muzdalifah before Mina there is a valley called Muhasser. This is the place where Abraha with his elephant army faced the stone rain by birds sent by Allah (SWT), in order to stop him from attacking Makkah. This whole event is mentioned in Surah Al-Feel. The masnoon way of crossing this place is in fast pace.