Prophet Muhammad (PBUH) said: “A group of people will come from the direction of Khorasan with a black flag. They will advance in such a way that no one will be able to stop them. Finally, they will reach Baytul Mudaddas (Jerusalem) and establish the Islamic Caliphate there.” Another hadith says: 'When you see the black flag coming from the direction of Khorasan, then join them even if you have to crawl on the snow, because among them will be the Caliph of Allah, the Mahdi..."