הלכה על סוטה 31:11

From its laws is that which they, may their memory be blessed, said (Sotah 16a), "How is the shaving? He passes the razor over his visible flesh - and even [on] the underarm and the pubic area and all the rest of the body - until he becomes [smooth] like a gourd, as it is stated (Leviticus 14:9), 'all of his hair.'" And when he shaves the two shavings, he only shaves with a razor. But if he shaved not with a razor - or he left two hairs - [it is as if] he has not done anything. And the rest of its details are in Tractate Negaim (see Mishneh Torah, Laws of Defilement by Leprosy 10).