Sunan Abi Dawud, Hadith 2582

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 ‏.‏

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also referenced under: Hadith 2576 (Hasan)