Assalam Alaikum
When someone has been harmed by the evil eye it means they have been harmed by a look from a Jinn.
I found some Ahadith concerning the evil eye.
Al-Muwatta Hadith
Hadith 50.1
Wudu from the Evil Eye
Yahya related to me from Malik that Muhammad ibn Abi Umama ibn Sahl ibn Hunayf heard his father say, "My father, Sahl ibn Hunayf did a ghusl at al-Kharrar. He removed the jubbah he had on while Amir ibn Rabia was watching, and Sahl was a man with beautiful white skin. Amir said to him, 'I have never seen anything like what I have seen today, not even the skin of a virgin.' Sahl fell ill on the spot, and his condition grew worse. Somebody went to the Messenger of Allah, may Allah bless him and grant him peace, and told him that Sahl was ill, and could not go with him. The Messenger of Allah, may Allah bless him and grant him peace, came to him, and Sahl told him what had happened with Amir. The Messenger of Allah, may Allah bless him and grant him peace, said, 'Why does one of you kill his brother? Why did you not say, "May Allah bless you?" (ta baraka-llah) The evil eye is true. Do wudu from it.' Amir did wudu from it and Sahl went with the Messenger of Allah, may Allah bless him and grant him peace, and there was nothing wrong with him."
Al-Muwatta Hadith
Hadith 50.2
Wudu from the Evil Eye
Malik related to me from Ibn Shihab that Abu Umama ibn Sahl ibn Hunayf said, ''Amir ibn Rabia saw Sahl ibn Hunayf doing a ghusl and said, 'I have not seen the like of what I see today, not even the skin of a maiden who has never been out of doors.' Sahl fell to the ground. The Messenger of Allah, may Allah bless him and grant him peace, was approached and it was said, 'Messenger of Allah, can you do anything about Sahl ibn Hunayf? By Allah, he can not raise his head.' He said, 'Do you suspect anyone of it?' They said, 'We suspect Amir ibn Rabia.' "
He continued, "The Messenger of Allah, may Allah bless him and grant him peace, summoned Amir and was furious with him and said, 'Why does one of you kill his brother? Why did you not say, "May Allah bless you?" Do ghusl for it.' Amir washed his face, hands, elbows, knees, the end of his feet, and inside his lower garment in a vessel. Then he poured it over him, and Sahl went off with the people, and there was nothing wrong with him."
Al-Muwatta Hadith
Hadith 50.3
Guarding from the Evil Eye.
Yahya related to me from Malik that Humayd ibn Qays al-Makki said, "A man came to the Messenger of Allah, may Allah bless him and grant him peace, with the two sons of Jafar ibn Abi Talib. He said to their nursemaid, 'Why do I see them so thin?' Their nursemaid said, 'Messenger of Allah, the evil eye goes quickly to them. Nothing stops us from asking someone to make talismans (using ayats of Qur'an) for them, except that we do not know what of that would agree with you.' The Messenger of ,Allah, may Allah bless him and grant him peace, said, 'Make talismans for them. Had anything been able to precede the decree, the evil eye would precede it.' "
Al-Muwatta Hadith
Hadith 50.4
Guarding from the Evil Eye.
Yahya related to me from Malik from Yahya ibn Said from Sulayman ibn Yasar that Urwa ibn az-Zubayr told him that the Messenger of Allah, may Allah bless him and grant him peace, entered the house of Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace. There was a child weeping in the house, and they told him that it was from the evil eye. Urwa said, "The Messenger of Allah, may Allah bless him and grant him peace, said, 'Why do you not find someone to make a talisman to protect it from the evil eye?' "
Sahih Muslim Hadith
Hadith 5427 Narrated byAbdullah ibn Abbas
Allah's Messenger (peace be upon him) said: The influence of an evil eyes is a fact; if anything would precede the destiny it would be the influence of an evil eye, and when you are asked to take bath (as a cure) from the influence of an evil eye, you should take bath.
Al-Tirmidhi Hadith
Hadith 1019 Narrated byAbuSa'id al-Khudri
The Prophet (peace be upon him) used to seek protection against the Jinn and the evil eye till surahs al-Falaq and an-Nas were revealed. After they were revealed he stuck to them and discarded everything beside them.
Transmitted by Tirmidhi.
End.
It is not a superstition it exists.
Assalam Alaikum