إِذَا رَأَى أَحَدُكُم مِنْ أَخِيهِ، أَوْ مِنْ نَفْسِهِ، أَوْ مِنْ مَالِهِ مَا يُعْجِبُهُ [فَلْيَدْعُ لَهُ بِالْبَرَكَةِ] فَإِنَّ الْعَيْنَ حَقٌّ
If you see something from your brother, yourself or wealth which you find impressing, then invoke blessings for it, for the evil eye is indeed true.
Ahmad 4/447, Ibn Majah, Malik. Al-Albani graded it authentic in Sahihul-Jami' As-Saghir 1/212. Also see Al-Arna'ut's checking of Ibn Al-Qayyim's Zadul-Ma'ad 4/170.