Musannaf Ibn Abi Shayba 28:1036

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Mujahid, who said: I saw Ibn Umar running quickly between the two targets in a qamis (shirt), and he was saying, "I am for it, I am for it," meaning when he hit [the target]. Then he would return, leaning on his bow, until he passed through the market.