Is it permissible to conceal a person's merits for fear of the evil eye, while reciting supplications, and has this been mentioned in a Hadith?

There is no harm in concealing a person's good qualities for fear of them being afflicted by the evil eye and envy, as this is a preventative measure. Ibn al-Qayyim mentioned that among the treatments and precautions against the evil eye is to conceal the good qualities of one who is feared to be afflicted by it, in a way that repels it. He cited the action of Uthman ibn Affan when he saw a beautiful boy and said: "Darken his dimple so that the evil eye does not strike him." There is no specific prophetic hadith (definition) concerning this matter.