Marshall Hodgson, in The Venture of Islam, has this to say on the topic: "The first Abbasi Caliph, al-Saffah, who happened to be the head of the Abbasid family at the moment when the Abbasi movement seized power in 750, set the Abbasi pattern chiefly in the sense that he slaughtered indiscriminately, treacherously, and, according to the accounts, with gross brutality as many members of the Umayyad family as he could lay hands on. He ordered that even the dead be desecrated. Among other tales is this: at one point he pretended to relent in his bloody search for Umayyad scions and invited all that remained to a banquet in token of forgiveness. Sitting at the meal, they were cut down by attendants; a carpet was spread over their dead and dying bodies and the banquet continued in the same room to the sound of their groans. The story is scarcely credible but illustrates what people thought of the Abbasid dynasty."

For what it's worth I know of no first hand or contemporary documentary account of the event. It is, as Hodgson writes, a tale or a legend. In that it also bears a strong resemblance to the supposed ways in which the Abbasid Caliphate itself was brought to an end. When the Mongols sacked Baghdad, as the most common version of the story goes, they didn't want to spill the blood of a ruler, so they rolled him up in a carpet and trampled him death. I much prefer a slightly more romantic version of the tale, which says that when the Mongols discovered the Caliph's treasury they locked him inside, letting him starve to death while contemplating whether he should have used his gold to build higher walls.

From an historiographical perspective the issue is basically that we just don't have good enough direct sources of the events in the 8th century to achieve this kind of accuracy. What we do know is that 1. the Abbasids killed lots of Umayyads 2. They didn't kill all of them, because they Umayyads then established a rival Caliphate in Cordoba.

It does make for a neat story, however.

I can't read the comments for some reason, but for those of you who have the book, it's on page 427. (I have the kindle edition)

Methodological point of order: Always always ALWAYS be suspicious of written sources, and anecdotes in particular. What text did the author of your secondary draw this story from? When was the text written and by whom? Is the story corroborated by other sources? If things don't add up, don't be afraid to throw up your hands and admit we have no idea.