What is the origin of the story that James had knees as hard as a camel's, because of his praying?

I'm reading John Pollock's "The Apostle - A Life Of Paul" and, at page 85 of the 1969 print, he states that > James was a man of such devotion that long praying had made his knees as hard as camel's, tradition says [...] He doesn't apply a note index for that. Anyone knows any source of that tradition?