What is the ruling on not washing the trailing part of the beard, and is washing an obligation such that ablution is invalidated by neglecting it?

Washing what hangs down from the beard during ablution is obligatory according to the preponderant opinion of scholars. So, whoever does not wash it, their ablution is invalid and must be repeated. Some scholars, however, hold the view that it is not obligatory, and in this case, the ablution is valid. The face extends from the hairline down to what hangs below the jawbones and chin in length, encompassing what hangs down from the beard both in width and length. Therefore, what hangs down from the beard is included within the boundaries of the face because it is part of what one faces with, and thus it must be washed like the rest of the face.