Al-Malik al-Mu’ayyad Abū al-Fidā’ Ismā‘īl b. ‘Alī al-Ḥamawī (d. 1331) , an important fourteenth-century historian from Mamluk Syria provides important insight into the practice of the cursing of Imām ‘Alī b. Abī Ṭālib (d. 661) by the Umayyads by demonstrating, on one hand, how this practice was in fact initiated by Mu’āwiya b. Abī Sufyān (r. 661-680) and, on the other, emphasizes the fact that it was carried on by all subsequent Umayyad caliphs until it was definitively abolished by the eighth Umayyad caliph, ‘Umar b. ‘Abd al-‘Azīz (r. 717-720). Although this fact had been mentioned by earlier historians, including Ibn Sa’d (d. 845), al-Ya ‘qubī (d. 897), al-Tabarī (d. 923) and al-Mas’udī (d. 956), this short excerpt reflects a late medieval Syrian (Sunni) Muslim narrative of this particular historical event.
Translation
‘Umar b. ‘Abd al-Azīz’s Abolishment of the Practice of Cursing ‘Alī b. Abī Ṭālib from the Pulpits:
It was customary for the Umayyad caliphs to publicly curse ‘Alī (may God be pleased with him) from the years 41 A.H. [661 A.D.], the same year that al-Ḥasan [b.’Alī] surrendered the caliphate, to the beginning of 99 A.H. [717 A.D.], in the last days of the caliphate of Suleymān b. ‘Abd al-Mālik. However, when ‘Umar b. ‘Abd al-‘Azīz became caliph, he abolished this practice and wrote to his governors and officials commanding them to abolish it. When he gave his Friday sermons, he would replace the cursing [of ‘Alī] that had ordinarily concluded the sermon with the following words of God Almighty: “Verily God enjoins justice, righteousness and good treatment of kith and kin, and forbids abomination, evil, and transgression. He admonishes you so that you may be mindful” [Q. 16:90]. After this day, ‘Alī was no longer publicly cursed from the pulpits and, instead, it became customary for all those giving the Friday sermon to recite this verse. Due to his actions in this regard, the caliph was highly praised by ‘Abd al-Rahmān al-Khuzā‘ī:
When you became caliph, you refused to curse ‘Alī and you did not perpetuate any vile practices.
You spoke truthfully and followed it with righteous action and for that you earned the praise of every Muslim”
[Abū al-Fidā’, al-Mukhtaṣar fī Akhbār al-Bashar (Cairo: Dar al-Ma’arif, 1998), Vol. 1, p. 250]
Ballandalus 
They did WHAT? Wow. All the stories that I read of the Sahaba kept their mouths firmly shut after the death of the Prophet, peace be upon him – I suppose because it’s easier for people to then venerate “The Prophet’s Generation” as faultless and blame sectarianism on “modern times”. It’s shocking to me to read of history post-Prophethood.