Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
Unintentional or intentional evil thoughts do not invalidate one's fast.
If the evil thoughts are involuntary, one will not be accounted for them. However, evil thoughts which are voluntary and taking enjoyment therefrom and having a firm drive to commit evil, is not permissible and one will be accounted for.
When one has evil thoughts, recite �Laa hawla walaa Quwwata Illaa Billaah� abaundantly, and ignore such thoughts. Do not pay attention to such thoughts and do not bother. Such evil thoughts being the whispers of Shaytaan should be treated with contempt and ignored. One should avoid seeing or doing anything that invite such thoughts. If one gives a little attention to such thoughts, the Shaytaan feels honoured. It has achieved its purpose of diverting and consuming one�s mind in evil and wrong. One should never give into that. Simply treat such thoughts with contempt and ignore them.
