Midrash su Salmi 119:114

Beloved are Israel, whom the Holy One Blessed be He surrounded with (seven) mitzvoth: Tefillin on the head and tefillin on the arm, mezzuzah on their doors, and tzitzith on the four corners of their garments. About them David said (Psalms 119:114) "(For these) seven in the day have I praised you." He entered the bathhouse and saw himself naked — whereupon he said: "Woe unto me that I am 'naked' of mitzvoth," but looking upon his circumcision, he uttered praise, viz. (Ibid. 12:1) "For the chief musician on the eighth, (circumcision, the eighth mitzvah, on the eighth day), a psalm of David."