Sayyiduna al-Hasan or Khilas narrates from Sayyiduna Ali (may Allah be pleased with him) (this doubt is Ibn Maymun's) that the Prophet (peace and blessings be upon him) said to Sayyiduna Ali (may Allah be pleased with him): "O Ali, I have entrusted to you this race among the people." Sayyiduna Ali (may Allah be pleased with him) went out, called Suraqah bin Malik and said: "O Suraqah, I place upon you the responsibility for the race that the Prophet (peace and blessings be upon him) placed upon me. When you reach al-mitar (the starting point of the course), line up the horses, then call out: 'Is there anyone adjusting a bridle, or mounting a boy, or throwing off a saddle-cloth?' When no one answers you, say the takbir three times, then release them at the third; and Allah will make whomever He wills of His creation fortunate by winning it." Sayyiduna Ali (may Allah be pleased with him) would sit at the end of the course and draw a line. He would stand two men facing each other at the edge of the line, its end between their big toes, and the horses would pass between the two men. He (may Allah be pleased with him) would say to the two of them: "When one horse comes out ahead of the other by the tips of its ears, or by one ear, or by the cheek-strap, award the race to it. If you are in doubt, split the prize between them in halves. When you pair two things (in a race), set the finish at the finish of the smaller of the two. And in racing there is 'la jalab wa la janab' (no jalab and no janab), and in Islam 'la shighar fi al-Islam' (there is no exchange marriage (shighar) in Islam)."
This chain is weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19781