Amīr al-mu’minīn’s warning to Anas ibn Mālik
Nahj al-Balāgha|Volume 1|Book 3|Chapter 311
فَقال (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.