Sahih Bukhari, Chapter: The Two Festivals (Eids)

I was with Ibn Umar when a spear head pierced the sole of his foot and his foot stuck to the paddle of the saddle and I got down and pulled his foot out, and that happened in Mina. Al-Hajjaj got the news and came to enquire about his health and said, "Alas! If we could only know the man who wounded you!" Ibn Umar said, "You are the one who wounded me." Al-Hajjaj said, "How is that?" Ibn Umar said, "You have allowed the arms to be carried on a day on which nobody used to carry them and you allowed arms to be carried in the Haram even though it was not allowed before."