It is narrated from Sayyiduna Abu Umamah al-Bahili (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Soon banners of Banu al-Abbas will come out from the east. The first of them will be doomed and the last of them will be doomed. Do not help them; may Allah not help them. Whoever walks under any of their banners, Allah will admit him to Hell on the Day of Resurrection. Listen! They are the worst of Allah's creation, and their followers are the worst of Allah's creation. They claim that they are from me. Listen! I am free of them and they are free of me. Their sign is that they will grow their hair long and wear black. So do not sit with them in gatherings, do not buy and sell with them in the markets, do not show them the way, and do not give them water to drink. The inhabitants of the heaven are hurt by the sound of their takbir."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7463