Sayyiduna Abdullah (bin Mas'ud) (may Allah be pleased with him) said: Umar bin al-Khattab (may Allah be pleased with him) rode a horse and urged it into a gallop, and his thigh became uncovered. The people of Najran saw a black mole on his thigh and said: "This is the one whom we find in our scripture, that he will expel us from our land."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 52