Muwatta Malik 1003

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّهُ سَمِعَ سَعِيدَ بْنَ الْمُسَيَّبِ، يَقُولُ لَيْسَ بِرِهَانِ الْخَيْلِ بَأْسٌ إِذَا دَخَلَ فِيهَا مُحَلِّلٌ فَإِنْ سَبَقَ أَخَذَ السَّبَقَ وَإِنْ سُبِقَ لَمْ يَكُنْ عَلَيْهِ شَىْءٌ ‏.‏

Yahya related to me from Malik that Yahya ibn Said heard Said ibn al-Musayyab say, "There is no harm in placing stakes on horses if a third horse enters it. The winner takes the stake, and there is no fine against the loser

Reference

Muwatta Malik 1003

In-book

Book 21, Hadith 45