Sunan Abi Dawud Book 15, Hadith 104
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.
The Prophet (ﷺ) said: If one enters a horse with two others when he is not certain that it cannot be beaten, it is not gambling; but when one enters a horse with two others when he is certain it cannot be beaten, it is gambling.
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.