The Messenger of Allah ﷺ said, "When you see the black banners coming from Khurasan, go to them, for among them is the Caliph of Allah, the Mahdi."
حدثنا وكيع عن شريك عن علي بن زيد عن أبي قلابة
قال رسول الله ﷺ إذا رأيتم الرايات السود قد جاءت من خراسان فأتوها فإن فيها خليفة الله المهدي
No references are available.
Its chain of transmission is daif. Ali bin Yazid's hadith is considered hasan when corroborated, but here he narrated it alone, as Ibn Adi said in 5/1783, and Dhahabi in Mizan 144.