From the Golden Legend.
“…Then one of the knights smote him as he kneeled before the altar on the head. And one Sir Edward Grim, that was his crossier put forth his arm with the cross to bear off the stroke, and the stroke smote the cross asunder and his arm almost off, wherefore he fled for fear, and so did all the monks, that were that time at compline. And then smote each at him, that they smote off a great piece of the skull of his head, that his brain fell on the pavement. And so they slew and martyred him, and were so cruel that one of them brake the point of his sword against the pavement. And thus this holy and blessed Archbishop S. Thomas suffered death in his own church for the right of all holy church. And when he was dead they stirred his brain, and after went in to his chamber and took away his goods, and his horse out of his stable, and took away his bulls and writings, and delivered them to Sir Robert Broke to bear into France to the king. And as they searched his chamber they found in a chest two shirts of hair made full of great knots, and then they said: Certainly he was a good man; and coming down into the churchward they began to dread and fear that the ground would not have borne them, and were marvellously aghast, but they supposed that the earth would have swallowed them all quick. And then they knew that they had done amiss. And anon it was known all about, how that he was martyred, and anon after took this holy body, and unclothed him, and found bishop's clothing above, and the habit of a monk under. And next his flesh he wore hard hair, full of knots, which was his shirt. And his breech was of the same, and the knots slicked fast within the skin, and all his body full of worms; he suffered great pain. And he was thus martyred the year of our Lord one thousand one hundred and seventy-one, and was fifty-three years old…”
Update: Here's the complete entry about St. Thomas from the Golden Legend.