Nahj al-Balāgha
Hadith 2187فَقال (عليه السلام): إِنْ كُنْتَ كَاذِباً فَضَرَبَكَ اللهُ بِهَا بَيْضَاءَ لَامِعَةً لَا تُوَارِيهَا الْعِمَامَةُ.
Thereupon, Amīr al-mu’minīn, peace be upon him, said: If you are speaking a lie Allāh may afflict you with white spots (leucoderma) which even the turban may not cover.