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 ‏.‏