Musnad Ahmad 32:1725

Ahmad ibn al-Hajjaj narrated to us, Hatim ibn Isma'il informed us, Mus'ab ibn Thabit ibn 'Abdullah ibn al-Zubayr narrated to us—he said: We sought knowledge regarding the wooden staff (al-'ud) that is in the place of the Imam, but we could not find anyone who could mention anything to us about it. Mus'ab said: Then Muhammad ibn Muslim ibn al-Sa'ib ibn Khabbab, the companion of the Maqsura (enclosure), informed me, saying: Anas ibn Malik sat with me one day and said: "Do you know why this was made?" I had not asked him about it, so I said: "No, by Allah, I do not know why it was made." Anas said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to place his right hand upon it, then he would turn to us and say: 'Straighten yourselves and level your rows.'"