I remember reading an article about it years back but I can't find anything to do with it now.
It's sort of like the OK sign, but when I look the OK sign up I find nothing to do with this older and perhaps entirely different gesture.
http://cihanhukumdari.istanbul.edu.tr/uploads/kanuni-sultan-suleyman-kimdir.jpg
Thanks in advance.
Have a read of this previous thread, which talks about the symbolism of the raised hand as it relates to Christian, Buddhist and Hindu iconography.
I'm not sure why specifically a Muslim ruler of Suleyman's period would be using it, whether it refers to Islamic symbolism (the hand of Fatima) or whether it's an imitation of European Christian portrait styles.