Plutarch writes about it in Life of Artaxerxes, chapter 16, and Wikipedia mentions that the Byzantine historian John Zonaras, who I'm not familiar with, talks about it too. It's really just a question of whether you believe them. I personally do; I'm not aware of any contradicting evidence (nor does it seem inherently unlikely), and while it's a good idea to be a bit skeptical of ancient historians, it doesn't strike me as a good policy to look at a colorful but feasible thing like scaphism, without any conflicting evidence, and just decide that Plutarch is making it all up.