SIKH REHAT MARYADA: Code of conduct on wearing head gear other than Turban - may be Cap / Hat

REHAT hoe sikh sir ttopee dhharai || saath janam kushattee hue marai || He who as a Sikh places a hat on their head; will be reborn seven times as a leper.  Thankhaanama Bhai Nand Laal Jee

GURBANI jo bin sathigur saevae khaadhae painadhae sae mueae mar ja(n)maekorrhae || oue haajar mit(h)aa boladhae baahar vis kadtehi mukh gholae || Those who eat and dress without serving the True Guru die; after death, those wretched lepers are consigned to reincarnation. In His Sublime Presence, they talk sweetly, but behind His back, they exude poison from their mouths.