Musar zu Wajikra 21:6

Commandment 265 forbids a priest who has purified himself -but still awaits sunset after ritual immersion- to perform sacrificial service. At that point he is still in the state of a secondary degree of ritual impurity. Concerning this the Torah writes: ולא יחללו שם אלוקיהם (21,6). Sanhedrin 83 explains that as the verse is not needed, because the information contained in it is already known, it is available to derive additional laws of a related matter, in this case the continued טומאה status of someone awaiting the sun to set after he has completed the ritual of purification. The similarity of the expressions חילול on both occasions lends weight to the validity of this rule.