Evil eye is real according to Islam

Evil eye is real and it can cause serious harm to the victim if they haven't sought protection to Allah.

Evil eye is not just a gaze/stare/look full of jealousy or intention of harm to the victim, it can even be a gaze at an object/thing/blessing with joy, excitement and pride. That is why, a Muslim is advised to keep reciting Mashallah La Quwwata Illa Billah" whenever he looks at the things he loves or sees with joy and pride (for example  your spouse, business, kids, health, cars, good attitude, good physique, friends, etc)

Simply put, any stare or gaze that is without the zikr/dhikr of Allah (the Creator of everything) becomes open to attack or infiltration from Shaitan (demons/satan/dark entities).

There is no question about jealousy-filled stares and envy as it definitely impacts the victim negatively, but good stares/gazes that are without the acknowledgement of Allah are also open to the effects of evil eye as shaitan infiltrates the hearts and causes harm when he sees that the victim is not protected by praising and appreciating Allah. (Always say in your heart and verbally: Mashallah la quwwata illa billa).

Evil eye has been mentioned in a number of authentic hadeeths. There are indirect references to evil eye in Quran as well. For instance in Surah Falaq, Allah instructs us to seek His protection from the evil of the envier when he envies.

Al Ain al Haq (Evil eye is real), an authentic hadith (mentioned in Ahmad, Al Tirmidhi and Muslim.

"The evil eye is real and if anything were to overtake the divine decree, it would be the evil eye. When you are asked to take a bath (for cure) from the effects of the evil eye, you should take a bath" 

From Quran and messengers lives
Quran mentions the story of Yaqub Peace Be Upon Him and his sons. The story tells us that Yaqub AS was worried about evil eye and that is why he did not want his all eleven sons not to enter Egypt together. Having eleven young and handsome sons itself was a great blessing of Allah and Yaqub AS was fully aware that people of Egypt could become jealous of the sons and might cast evil eye out of envy.