Yahya related to me from Malik that Yahya ibn Said heard Said ibnal-Musayyab say, "There is no harm in placing stakes on horses if athird horse enters it. The winner takes the stake, and there is nofine against the loser." (DE)
USC-MSA web (English) reference: Book 21, Hadith 46 (DE)
Arabic reference (DE): Book 21, Hadith 1006 (DE)