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?
The first recorded instance of the tradition comes from Hegesippus, a second century Christian writer.  Unfortunately, his works have been lost, except for a small portion of his writings quoted by later authors.
In his Church History (c. 325), Eusibius writes:

But Hegesippus, who lived immediately after the apostles, gives the most accurate account in the fifth book of his Memoirs. He writes as follows:
... [James] alone was permitted to enter into the holy place; for he wore not woolen but linen garments. And he was in the habit of entering alone into the temple, and was frequently found upon his knees begging forgiveness for the people, so that his knees became hard like those of a camel, in consequence of his constantly bending them in his worship of God, and asking forgiveness for the people.

In his book, James the Brother of Jesus, Robert H. Eisenman states that this image is &quot;so original that it is difficult to imagine that Hegesippus simply made it up.&quot;  In other words, the story likely either has some basis in actual history or at least developed from a tradition earlier than Hegesippus' time.  Thus, the origin of the story can likely be pushed back to at least the early second century.