"No harm shall be inflicted or reciprocated." لا ضرر ولا ضرار
This principle is actually one of the prophet's Hadiths. The wording refers to both physical and psychological harm. Another narration of the same Hadith adds: "Whoever causes harm, Allah will harm him, and whoever causes distress, Allah will distress him." The principle of not harming is also emphasized by many ayat of the Qur'an such as "...and do not harass them (your women) to make them miserable" [65:6]. The Qur'an also prohibits parents from using their children to cause harm to each other.
It was on the basis of this principle that the prophet gave permission to one companion to cut down the tree of a man who used to cause harm because of his tree. The principle dictates that a water well owner must give his neighbors free access to the water until they get their own well. More examples are found in the biographies of the prophet and the righteous Muslim leaders after him.