He reported God's Messenger as saying, “If one enters a horse with two others there is no good in it if he is certain it cannot be beaten, but there is no harm if he is not." It is transmitted in Sharh as-sunna. In Abu Dawud’s version he said, “If one enters a horse with two others when he is not certain that it cannot be beaten, it is not gambling; but when one enters a horse with two others when he is certain it cannot be beaten, it is gambling."
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ أَدْخَلَ فَرَسًا بَيْنَ فَرَسَيْنِ فَإِنْ كَانَ يُؤْمَنُ أَنْ يَسْبِقَ فَلَا خَيْرَ فِيهِ وَإِنْ كَانَ لَا يُؤْمَنُ أَنْ يَسْبِقَ فَلَا بَأْسَ بِهِ» . رَوَاهُ فِي شَرْحِ السُّنَّةِ وَفِي رِوَايَةِ أَبِي دَاوُدَ: قَالَ: «مَنْ أَدْخَلَ فَرَسًا بَيْنَ فَرَسَيْنِ يَعْنِي وَهُوَ لَا يَأْمَنُ أَنْ يَسْبِقَ فَلَيْسَ بِقِمَارٍ وَمَنْ أَدْخَلَ فَرَسًا بَيْنَ فَرَسَيْنِ وَقَدْ أَمِنَ أَنْ يَسْبِقَ فَهُوَ قمار»
sunnah.com