Ibn Al-Qayyim said:
The origin of the evil eye is taking a liking to something, then the evil soul follows it, pursues it and seeks to do harm to it, seeking help to apply its poison by looking at the object. Zaad Al-Ma'aad. by Ibn Al-Qayyim, 4/167
Ibn Manzoor has said: It is said "So-and-so was struck by the evil eye" if an enemy or an envious person looks at him and he is affected by that and becomes sick as a result.”
Lisaan Al-Arab by Ibn Manzoor, 13/301
Al-Hafiz ibn Hajar has said: “What is meant by the evil eye is looking at something with admiration contaminated with envy from one who is evil in nature so it results in harm.”
Fath Al-Baari, by Ibn Hajar, 10/200