Hadith 669
Muhammad bin Muslim bin al-Saib said: one day I prayed by the side of Anas bin Malik. He said ; Do you know why this stick is placed here ? I said: No, by Allah. He said; The Messenger of Allah ﷺ used to put his hand upon it and say: Keep straight and straighten your rows.
Hadith 670
This tradition has also ben transmitted by Anas through a different chain of transmitters. This version goes: when the Messenger of Allah ﷺ stood for prayer, he took it (the stick) in his right hand and turning (to the right side) said; keep straight and straighten your rows. He then took it in his left hand and said; keep straight and straighten your rows.