In a previous posting, we asked
Who Said it? Tzitzis on Display
Who wrote that the (translation is mine) Main (Iqqar) mitzva of Tallith Qatan is to wear it ON one's clothes in order to always wear it and remember the Mitzvos?
The answer is:
Shulchan Aruch (of R. Y. Caro)
Orach Hayyim 8:11