Sahih al-Bukhari Book 78, Hadith 74
Allah's Messenger (ﷺ) said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr
(disbelief).
The Prophet (ﷺ) said at Mina, "Do you know what day is today?" They (the people) replied, "Allah and His Apostle know better," He said "Today is 10th of Dhul-Hijja, the sacred (forbidden) day. Do you know what town is this town?" They (the people) replied, "Allah and His Apostle know better." He said, "This is the (forbidden) Sacred town (Mecca a sanctuary)." And do you know which month is this month?" They (the People) replied, "Allah and His Apostle know better." He said, ''This is the Sacred (forbidden) month ." He added, "Allah has made your blood, your properties and your honor Sacred to one another (i.e. Muslims) like the sanctity of this day of yours in this month of yours, in this town of yours." (See Hadith No. 797, Vol. 2.)
That he heard the Prophet (ﷺ) saying, "If somebody accuses another of Fusuq (by calling him 'Fasiq' i.e. a wicked person) or accuses him of Kufr, such an accusation will revert to him (i.e. the accuser) if his companion (the accused) is innocent."