From the Greatest of Filth in Hearts
Imām Ibn Taimiyyah, may Allāh have mercy on him, said:
From the greatest of filth [to be found] in hearts is [that which] occurs of hatred and malice in the heart of a slave toward the best of believers, the heads amongst Allāh’s most pious believers, after the prophets.
Minhāj Al-Sunnah Al-Nabawiyyah, vol. 1, p. 22.