Is there a ḥadīth that proves that it was written on the forehead of Ibn Sayyād "disbeliever" (kāfir) or "k f r"? And is Ibn Sayyād a jinn or a human, given that he swelled up with rage?

There is no Hadith that proves that "Kafir" (disbeliever) was written on the forehead of Ibn Sayyad, and he is not the promised Antichrist (Dajjal al-Akbar). Rather, he was a dajjal (impostor/deceiver) from among the Children of Adam and embraced Islam after the passing of the Prophet, may Allah's prayers and peace be upon him. Ibn Sayyad denied being the Antichrist, reasoning that the descriptions of the Antichrist mentioned in the Ahadith did not apply to him, such as being a disbeliever, barren, or forbidden from entering Mecca and Medina. He told Abu Sa'id al-Khudri that he was a Muslim, had children, and had entered Mecca and Medina. Ibn Sayyad died in Medina as a Muslim. His described swelling in the Hadith in Sahih Muslim might be among the supernatural phenomena that occur to charlatans.