Hadith.766 - Imam Jafar ibn Muhammad Al-Sadiq (as) was asked about praying while wearing a black cap.
Imam (as) said: "Do not pray in it, for it is the attire of the people of the Fire”.
Al-Kāfi #1577“I once asked abu ’Abd Allah (a.s.), if I can perform Salah (prayer) with a black cap. He (the Imam) said, ‘Do not perform Salah (prayer)...