ENOfficial
Thauban reported God's messenger as saying, "When you see the black standards have come from the direction of Khurasan go to them, for God's khalifa the Mahdi will be among them." Ahmad and Baihaqi, in Dala'il an-nubuwa, transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ ثَوْبَانَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِذَا رَأَيْتُمُ الرَّايَاتِ السُّودَ قَدْ جَاءَتْ مِنْ قِبَلِ خُرَاسَانَ فَأْتُوهَا فَإِنَّ فِيهَا خَلِيفَةَ اللَّهِ الْمَهْدِيَّ» . رَوَاهُ أَحْمَدُ وَالْبَيْهَقِيُّ فِي «دَلَائِل النبوَّة»
sunnah.com