Controlling the tongue and the Prophet’s saying: “Speak good or remain silent.”

Narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever guarantees me (the proper use of) what is between his jaws and what is between his legs, I guarantee Paradise for him." (i.e., he never misuses these two in the rights of Allah or the rights of people.)

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "A person may utter a word that pleases Allah, without giving it any importance, and because of that Allah raises his ranks. And (sometimes) a person may utter a word that angers Allah, and he does not consider it a major sin, yet because of that he falls into Hell."