This is what the verse said: “Do not allow your mouth to bring sin upon your flesh” (Ecclesiastes 5:5). The Rabbis say: This verse is speaking of those who engage in malicious speech. How so? “Do not allow your mouth.”
What does this mean? Once the mouth [of a person] utters malicious speech, it has committed a sin against his body, in that it causes it to be afflicted [by leprosy]. That is the meaning of: “to bring sin upon your flesh,” as the mouth commits a sin against the flesh. What is meant by, “and do not say before the angel that it was a mistake”? (Ecclesiastes 5:5).
It means that you should not say: ‘I will go and utter malicious speech and no person will know about it.’ The Holy One blessed be He says to him: ‘Know that I will send an angel who will stand alongside you and record everything that you say about another person.’ From where is this derived? It is as it is stated: “Even in your thoughts, do not curse a king” (Ecclesiastes 10:20).
Why? “Because a bird of the heavens will carry the voice [and a winged creature will tell the matter]” (Ecclesiastes 10:20). What is “and a winged creature will tell the matter”? (Ecclesiastes 10:20). This refers to the angels in whose regard it is written: “Six wings to each one” (Isaiah 6:2).
“Why should God become angry at your voice” (Ecclesiastes 5:5) – at that voice that emerged from your mouth. “And destroy your handiwork” (Ecclesiastes 5:5) – as that person will be afflicted with leprosy. If you do not believe it, consider Miriam, who, because she uttered malicious speech about Moses her brother, was afflicted. “Remember what the Lord your God did…” and as a result, she was afflicted.