Whoever keeps a horse in the path of Allah out of faith in Allah and belief in His promise, its feed, drink, dung and urine will be good deeds in his Balance

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever keeps a horse in the way of Allah out of faith in Allah and affirming His promise, then that horse's eating its fill, its drinking its fill, its dung and its urine will all be in the scale of his deeds as good deeds on the Day of Resurrection."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Every Arabian horse is granted permission every day to make two supplications. It says: 'Allahumma kama khawwaltani man khawwaltani, faj'alni min ahabbi malihi wa ahlihi ilayh' (O Allah, just as You have placed me in the ownership of this master, so make me the most beloved to him of his wealth and his family)."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abu Qatadah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The best horse is one that is deep black, with a white mark on its forehead, three white feet and a white lower lip, while its right foreleg is free (of whiteness). If such a black one is not available, then a reddish-black one (kumayt) with these same markings."
This is a gharib (singular) sahih (authentic) hadith; the two Shaykhs (Bukhari and Muslim) used all of its narrators as evidence, but they did not record it.