From the workshop of Alessandro Filipepi, called Sandro Botticelli (1445 – 1510), a "fragment" from an oil on panel of Saint John the Baptist. I haven't found an image of the full work from which this was cropped.
From a private collection, via Vision and Alabaster.
Addendum: Ryan Beales found a virtually identical John the Baptist in a different piece from the same workshop; it's in the National Gallery.