In societies that lack procedure for peaceful power transfer—which was nearly everyone in the 8th century—it’s important to move fast to cement your new power. It’s even better for the people, who probably have little contact with the rulers. If you leave things uncertain, there will be civil war, and that’s bad for everyone, but most of all for the poor sods whose farms get trashed while their sons are sent out to die. Somebody has to die; it will be a strategic few or a randomly-chosen many. All this throat-clearing leads up to the next step for the Abbasids: killing the Umayyads.
The wealthy families of the top Umayyad princes lived in isolated but well-known places. The cities may have been sweating on the riverbank or plain, but the houses of the rich have always been located in the cool hills near a fast-moving creek or artesian spring. An Umayyad palace was a place of palm trees, fountains, grass and flowers. The children grew up riding horses and swimming in safe pools, learning the education tools of ruling. They lived in tents that sentimentally reminded former nomads of the old life but actually had many luxuries.
When the Abbasids secured Damascus, a detachment of men rode fast to the main collection of country houses to the north, near al-Resafa. Without warning, they invaded the tents and began killing. Any child who survived, especially a boy, could be used as the figurehead of a future counter-revolution and civil war.
Two brothers, one aged 20, the other a bit younger, were on the outskirts of the compound with the four-year-old son of the older brother. With a servant named Bedr, and maybe with some of their sisters, they broke away from the compound and ran. They fled to a village on the Euphrates riverbank, where some people hid them. But their absence was noticed, and their flight was tracked.
A squad of soldiers came to the village to search for them. Escape was high risk, but the brothers decided to leave the child behind (amazingly, he survived) and try to swim the river. The older brother and the servant crossed the river, but the younger brother grew tired and was persuaded to turn back. The pursuers were shouting to them, “Come back! We will spare your lives!” Of course, they didn’t, so the younger brother died. But the two who came out on the other side continued to run.
Prince Abd al-Rahman and his servant Bedr successfully dodged Abbasid search parties for weeks. They made their way back across the Euphrates River, through Syria and Israel, southward into Egypt. Doubtless it was dangerous and terribly uncomfortable, as they must have hid in the bush or desert for days at a time with little to eat, traveling by night, going from hut to hut never knowing if they would be betrayed, trying not to be recognized.
Abd al-Rahman had a definite goal in his mind. Although a grandson of Caliph Hisham, he was the son of a Berber woman, probably a high-ranking slave sent from North Africa by Musa ibn Nusayr. She had spoken Berber/Amazigh language to her son, although of course he spoke Arabic with the rest of the family. Abd al-Rahman was making a beeline for the land of his mother’s tribe, to claim her name and raise his own army.