Tazria 

תזריע

The commandment of circumcision is especially dear: The result of the mitzva is impressed upon the body of the man himself, and accompanies him everywhere, and at all times. 

When David entered the bathhouse and saw himself standing naked, he said: Woe is me that that I am standing naked, without any mitzva. Once he remembered the circumcision in his flesh, his mind was put at ease, as he realized he was still accompanied by this mitzva. After he left the bathhouse, he recited a song about the mitzva of circumcision, as it is stated: “For the chief musician, on the eight-stringed harp [hasheminit], a psalm by David” (Psalms 12:1). This is a reference to circumcision, which was given to be performed on the eighth [bashemini] day. <source>(Menaĥot 43b) </source>