Sayyiduna Imran bin Husayn (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "In a horse race, al-jalab (keeping someone behind one's horse to keep urging it to run fast) and al-janab (keeping someone with a horse at one's side, so that if the first horse grows tired one can mount the other) are not permitted in the wager."
(b) In the narration of Humayd it is: "Al-jalab, al-janab and al-shighar (that is, exchange marriage) are not permissible in Islam."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19778