If you want to prevent the Umayyads from coming to power, than you need to prevent Uthman from becoming Caliph.
The Umayyads were the dominate power in Mecca before Muhammad conquered Mecca. Afterwards, the Prophet dismantled the Umayyad's power structure, and gave control of the Kaaba to his own clan, the Hashemites. After the death of Caliph Umar, a council of six was chosen to elect a new Caliph. Both Ali and Uthman were on this council. Each of them were asked who they would vote for. When Ali was asked, he abstained, while Uthman voted for Ali. If Ali had voted for himself instead of abstaining, than he would have been elected Caliph. But IOTL Uthman was elected, and during his reign he indulged in a lot of nepotism, appointing his close relatives as governors across the Caliphate, which put the Umayyads in position to revolt against Ali when he was elected after Uthman's murder.
The PoD is simple. Have Ali vote for himself instead of abstaining after the death of Umar. The difficulty with this, however, is that Ali is young. He would have been 43 when elected. This means that he would probably have a long reign, if he doesn't step on too many toes, and brings in as many elders into his decision making process as possible.