Cat Stevens didn’t, but Yusuf Islam did...
He wrote something shortly after 9/11 that praised the terrorists. Fortunately, he seems to have had the good sense to shut up after that, and it’s all been forgotten.
OK, I just checked out his Wikipedia page. He said some controversial stuff about Salmon Rushdie - he “clarified”, retracted or restated those things (yes, it is controversial), but in regards to 9/11
Wikipedia says:
Immediately following the 11 September 2001, attacks on the United States, he said:
I wish to express my heartfelt horror at the indiscriminate terrorist attacks committed against innocent people of the United States yesterday. While it is still not clear who carried out the attack, it must be stated that no right-thinking follower of Islam could possibly condone such an action. The Qur’an equates the murder of one innocent person with the murder of the whole of humanity. We pray for the families of all those who lost their lives in this unthinkable act of violence as well as all those injured; I hope to reflect the feelings of all Muslims and people around the world whose sympathies go out to the victims of this sorrowful moment.