How did the Prophet, peace and blessings be upon him, suspect Ibn Sayyad of being the Antichrist (Al-Masih Ad-Dajjal), knowing that the word "Kafir" (disbeliever) would be written on his forehead? And will this writing appear when he emerges, or has it been present since his creation?

The Prophet, peace and blessings be upon him, sought refuge from the trial of the Antichrist (Al-Masih Ad-Dajjal). Allah has placed between the eyes of the Dajjal the word "Kafir" (disbeliever), which every believer will read. This writing is real and will appear at the time of the Dajjal's trial, when he claims divinity, and not before then. As for Ibn Sayyad, the Prophet, peace and blessings be upon him, did not definitively state that he was the Dajjal, nor had he received a revelation regarding the appearance of the mark of disbelief between Ibn Sayyad's eyes. Its appearance will only occur upon the Dajjal's emergence and his trial.