Sunan Abi Dawud Book 15, Hadith 105
The Prophet (ﷺ) said: There must be no shouting or leading another horse at one's side. Yahya added in his tradition: When racing for a wager.
The tradition mentioned above has also been transmitted by Al Zuhri with the chain of ‘Abbad and to the same affect.
Abu Dawud said “This tradition has also been narrated by Ma’mar, Shu’aib and ‘Aqil on the authority of Al Zuhri from a number of scholars and this is the soundest one in our opinion.
Qatadah said “Taking another horse behind one’s horse to urge it on and taking another horse at one’s side are (done) in a horse race.