Staying at Al-Mahassab is not one of the ceremonies (of Hajj), but Al-Mahassab is a place where Allah's Messenger (ﷺ) camped (during his Hajjat-al-Wida).
Sahih al-Bukhari Book 25, Hadith 242
← Previous Hadith It (i.e. Al-Abtah) was a place where the Prophet (ﷺ) used to camp so that it might be easier for him to depart.

Sahih al-Bukhari Book 25, Hadith 241 Next Hadith → Ibn `Umar used to spend the night at Dhi-Tuwa in between the two Thaniyas and then he would enter Mecca through the Thaniya which is at the higher region of Mecca, and whenever he came to Mecca for Hajj or `Umra, he never made his she camel kneel down except near the gate of the Masjid (Sacred Mosque) and then he would enter (it) and go to the Black (stone) Corner and start from there circumambulating the Ka`ba seven times: hastening in the first three rounds (Ramal) and walking in the last four. On finishing, he would offer two rak`at prayer and set out to perform Tawaf between Safa and Marwa before returning to his dwelling place. On returning (to Medina) from Hajj or `Umra, he used to make his camel kneel down at Al-Batha which is at Dhul-Hulaifa, the place where the Prophet used to make his camel kneel down.

Sahih al-Bukhari Book 25, Hadith 243