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.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ جَلَبَ وَلاَ جَنَبَ زَادَ يَحْيَى فِي حَدِيثِهِ فِي الرِّهَانِ  

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.  

الْجَلَبُ وَالْجَنَبُ فِي الرِّهَانِ