Hajj (Pilgrimage) -
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).
AI Summary
The key points are:
1. A woman from the tribe of Khath'am asked the Prophet (ﷺ) if she could perform the Hajj pilgrimage on behalf of her old and weak father, and the Prophet (ﷺ) allowed her to do so.
2. The Prophet (ﷺ) would start saying "Labbaik" (here I am) when his mount stood upright at Dhul Hulaifa before the Hajj.
3. The Prophet (ﷺ) would enter the state of Ihram (ritual consecration) for Hajj at Dhul Hulaifa when his mount became steady.