Khutbah - the primary formal occasion for public preaching in the Islamic tradition
I actually was there when he did this. I remember this really made me stop to evaluate my life. We aren't only responsible for what we do (prayers, zakah, Hajj), but we're also responsible for how we treat people around us.
To quote HP: "If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals." - Arthur (HP book 2)