Qatadah (may Allah have mercy on him) said: In wagering on a horse race, al-jalab (that is, keeping someone behind to make the horse run fast) and al-janab (that is, keeping someone with a horse alongside so that it can be ridden if the first one tires) are not permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19779