What is the ruling on someone who accuses a person of being "undutiful toward the right of Allah" or a "liar" without verifying that the accusation is true, and what is the evidence for this from the Qur'an and Sunnah?

Harmful behavior towards Muslims and insulting them without justification are among the greatest prohibitions. Allah has warned those who cause harm, saying: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin" (Quran 33:58), and His saying: "Woe to every slanderer and backbiter" (Quran 104:1). The Prophet, peace and blessings be upon him, forbade such acts, saying: "All of a Muslim is inviolable for another Muslim: his honor, his wealth, and his blood," and "Insulting a Muslim is wickedness, and fighting him is disbelief," and "Whoever calls a man a disbeliever or says 'enemy of Allah,' and he is not so, then it will recoil on him." Therefore, insulting a believer and speaking about his honor unjustly is forbidden by the consensus of the Ummah, and its perpetrator is a transgressor, unless he repents and reforms. The sin becomes even greater when the insult is a lie.