Deuteronomy 23:9-11

When the host goeth forth against thine enemies, then keep thee from every wicked thing. If there be among you any man, that is not clean by reason of uncleanness that chanceth him by night, then shall he go abroad out of the camp, he shall not come within the camp: but it shall be, when evening cometh on, he shall wash himself with water: and when the sun is down, he shall come into the camp again.