Mahsab is nothing significant. It is merely a place where the Messenger of Allah ﷺ stopped to rest.
حدثنا سفيان عن عمرو عن عطاء
ليس المحصب بشيء إنما هو منزل نزله رسول الله ﷺ
Its isnad is sahih. Muhassab, with the doubled and open sad, is a place between Makkah and Mina, closer to Mina. The Messenger of Allah ﷺ camped there because it made his departure easier, and it is not one of the sunnahs of Hajj. Bukhari and Muslim also narrated this hadith, as noted in al-Muntaqa 2659. See also what comes at 3289.