Background: Yusuf Islam (born Steven Demetre Georgiou; 21 July 1948), commonly known by his former stage name Cat Stevens, is a British singer-songwriter and multi-instrumentalist. His 1967 debut album reached the top 10 in the UK, and the album's title song "Matthew and Son" charted at number 2 on the UK Singles Chart. Stevens' albums Tea for the Tillerman (1970) and Teaser and the Firecat (1971) were both certified triple platinum in the US by the RIAA. His musical style consists of folk, pop, rock, and Islamic music.
Context: Immediately following the September 11 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.  He appeared on videotape on a VH1 pre-show for the October 2001 Concert for New York City, condemning the attacks and singing his song "Peace Train" for the first time in public in more than 20 years, as an a cappella version. He also donated a portion of his box-set royalties to the fund for victims' families, and the rest to orphans in underdeveloped countries. During the same year, he dedicated time and effort in joining the Forum Against Islamophobia and Racism, an organisation that worked towards battling misconceptions and acts against others because of their religious beliefs or their racial identity (or both), after many Muslims reported a backlash against them due in part to the grief caused by the events in the United States on 11 September.
Question: What is significant about September 11 for Cat Stevens?. Whats the answer?
He appeared on videotape on a VH1 pre-show for the October 2001 Concert for New York City, condemning the attacks and singing his song "Peace Train