Hadith 15:106
Sahih Maqtu' (Al-Albani)
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.
حَدَّثَنَا ابْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الأَعْلَى، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، قَالَ الْجَلَبُ وَالْجَنَبُ فِي الرِّهَانِ
Original Reference: Sunan Abi Dawud 2582