And the servants of the Most Merciful are those who walk upon the earth easily, and when the ignorant address them [harshly], they say [words of] peace (25:63)

If someone verbally abuses a person who is fasting, then he should not respond by swearing back. Instead, he should say: I am fasting.

Akhlaq e Hasanah (good akhlaq) urges us to respond evil with goodness, instead of responding with evil.

Things to do:

If someone quarrels with a person who is fasting, then it is better to have patience and to not fight back. One should respond by saying: I am fasting.

Prophet of Allahﷺ has forbidden taking revenge while fasting.