An Evil Eye
It is forbidden to stand at your neighbor's field, to gaze at it at the time the crops are standing, so as not to harm them with an evil eye. It is certainly forbidden to gaze at someone in a way that might damage him personally as a result of an evil eye. Even with regard to his business and occupation where there is no cause to fear an evil eye, if he works in his own house or property, it is forbidden to watch him without his knowledge, for he may not want others to know of his business and occupation. It is good manners, when seeing someone at his work, to bless him and say, "May you be successful at your task."
(Kitzur Shulchan Aruch 183:6)