What is the ruling on taking something from the remnants of an envier without his knowledge, and using it as incense to fumigate oneself with, in order to repel the evil eye?

It is not permissible to accuse anyone of the evil eye without proof.

The benefit of fumigation with the traces of the person who cast the evil eye is not known in Islamic law nor by reason. Therefore, anything people do to ward off the evil eye, which has no basis in Islamic law or divine decree, is not permissible. Sheikh al-Islam Ibn Taymiyyah mentioned that fumigating homes is a custom of Christians and Sabians, and it is an act of sorcerers who deal with jinn.

The religiously prescribed treatment for the evil eye is for the person who cast the evil eye to perform ghusl (ritual washing) – by washing his face, hands, elbows, knees, the ends of his feet, and the inside of his lower garment in a vessel – then pour the water over the afflicted person. As for taking from the waste products or traces of the person who cast the evil eye, there is no basis for it.

If the person who cast the evil eye is not known or does not comply, then recourse to ruqyah (Islamic incantations) is the solution. The Prophet, peace and blessings be upon him, commanded seeking ruqyah for the evil eye. Among the valid ruqyah is for the Muslim to say: "Bismillahi arqika, min kulli shayin yu'dhika, min sharri kulli nafsin aw 'aynin hasidin Allahu yashfika, Bismillahi arqika" (In the Name of Allah, I perform ruqyah for you, from everything that harms you, from the evil of every soul or envious eye, may Allah cure you, in the Name of Allah, I perform ruqyah for you).