It's mentioned in chapter 602 page 17 that Madara gave away his eye to someone... But it's not mentioned whom. He also says that his eyes aren't his own (he could be referring to his brother's eyes). Most of the Q&A discussions I came across on Google say that it's donated to Nagato! It's mentioned in Wiki too, but I haven't come across any anime episode or manga chapter stating it. Has it ever been mentioned anywhere else, to whom did he donate his eyes (or at least his right eye)?