“During the reigns of the Umayyad Caliphs his blessed resting-place could not be disclosed, and so it was also under the Abbasids until the reign of Harun al-Rashid. But in the year 175 A.H. (791 A.D.), Harun happened to go hunting in these parts, and the deer he was chasing took refuge on a small piece of raised ground. However much he asked his hunting dogs to capture the quarry, they refused to go near this spot.
He urged his horse to this place, and the horse too refused to budge; and on this, awe took possession of the Caliph’s heart, and he immediately started to make inquiries of the people of the neighborhood, and they acquainted him with the fact that this was the grave of Imam Ali ibn Abu Talib, the cousin and son-in-law of the Holy Prophet. Harun ordered a tomb to be erected over the grave, and people soon began to settle down in its vicinity.”
The Shrine of Ali at Najaf from “The Shi’ite Religion” by Dwight M. Donaldson.