What is the prophetic treatment for the evil eye?

Two remedies are established in the Sunnah.

The washing of the one who looked

Where the person responsible is known and willing, the Prophet, peace be upon him, instructed that they perform a specific washing, and that the water used be poured over the affected person. The narration involving Sahl ibn Hunayf records exactly this, and the man recovered.

This is the most direct treatment, though in practice the person is rarely known.

Ruqya

Where they are not known, recitation is the remedy. Recite Al-Fatiha, Ayat al-Kursi, and the three protective surahs over the affected person, along with the prophetic dua asking Allah to remove the harm, the Lord of mankind, healing that leaves no illness behind.

Practitioners commonly recite over water for the person to drink and bathe with over several days.

Effects from the eye typically lift faster than sihr, often within days rather than months. Where symptoms persist for weeks despite treatment, that is a signal to look at other explanations, including medical ones.