What is the ruling on a Muslim saying "You disbeliever" to his brother, and does the one who says it become a disbeliever according to Imam Malik, especially if he fears disbelief if he does not declare others disbelievers?

The issue of takfir (declaring someone an apostate) is grave. Whoever says to his brother, "O Kafir!" without legitimate Sharia evidence, has incurred a great sin. The Messenger of Allah, peace and blessings be upon him, said: "Any man who says to his brother, 'O Kafir,' then it has returned upon one of them; if it is as he said, otherwise it has returned upon him." According to the madhhab of Imam Malik, this does not constitute apostasy that expels one from the fold of Islam. Scholars interpret the hadith to mean that the speaker bears the burden of this statement and its slander, and it does not signify actual apostasy for the speaker, unless his intention was to deem that permissible, or if he was from the Khawarij who declare Muslims apostates for sins. The hadith forbids declaring a Muslim an apostate or a transgressor, as in the Almighty's saying: "Nor call each other by [offensive] nicknames."