I typically read that CAT IIIB approaches must be flown by an autopilot to touchdown. Looking for a reference (regulatory bodies* in particular) to track the statement only leads to company SOPs.
In other words, with the runway in sight at or above the DH, is it written somewhere (excl. SOPs) that the pilot flying can/can't take manual control when the approach was started in CAT IIIB minima?
Full disclosure: I completely understand it's unwise and unsafe to even suggest hand flying an approach when that's the view:
RVR 200 m. (wikimedia.org)
* FAA, EASA, and ICAO, are acceptable (I couldn't find anything in ICAO Annex 6).