وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِي صَالِحٍ السَّمَّانِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ الْخَيْلُ لِرَجُلٍ أَجْرٌ وَلِرَجُلٍ سِتْرٌ وَعَلَى رَجُلٍ وِزْرٌ فَأَمَّا الَّذِي هِيَ لَهُ أَجْرٌ فَرَجُلٌ رَبَطَهَا فِي سَبِيلِ اللَّهِ فَأَطَالَ لَهَا فِي مَرْجٍ أَوْ رَوْضَةٍ فَمَا أَصَابَتْ فِي طِيَلِهَا ذَلِكَ مِنَ الْمَرْجِ أَوِ الرَّوْضَةِ كَانَ لَهُ حَسَنَاتٌ وَلَوْ أَنَّهَا قَطَعَتْ طِيَلَهَا ذَلِكَ فَاسْتَنَّتْ شَرَفًا أَوْ شَرَفَيْنِ كَانَتْ آثَارُهَا وَأَرْوَاثُهَا حَسَنَاتٍ لَهُ وَلَوْ أَنَّهَا مَرَّتْ بِنَهَرٍ فَشَرِبَتْ مِنْهُ وَلَمْ يُرِدْ أَنْ يَسْقِيَ بِهِ كَانَ ذَلِكَ لَهُ حَسَنَاتٍ فَهِيَ لَهُ أَجْرٌ وَرَجُلٌ رَبَطَهَا تَغَنِّيًا وَتَعَفُّفًا وَلَمْ يَنْسَ حَقَّ اللَّهِ فِي رِقَابِهَا وَلاَ فِي ظُهُورِهَا فَهِيَ لِذَلِكَ سِتْرٌ وَرَجُلٌ رَبَطَهَا فَخْرًا وَرِيَاءً وَنِوَاءً لأَهْلِ الإِسْلاَمِ فَهِيَ عَلَى ذَلِكَ وِزْرٌ ‏"‏ ‏.‏ وَسُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنِ الْحُمُرِ فَقَالَ ‏"‏ لَمْ يَنْزِلْ عَلَىَّ فِيهَا شَىْءٌ إِلاَّ هَذِهِ الآيَةُ الْجَامِعَةُ الْفَاذَّةُ ‏{‏فَمَنْ يَعْمَلْ مِثْقَالَ ذَرَّةٍ خَيْرًا يَرَهُ وَمَنْ يَعْمَلْ مِثْقَالَ ذَرَّةٍ شَرًّا يَرَهُ ‏}‏‏.‏

Yahya related to me from Malik from Zayd ibn Aslam from Abu Salihas-Samman from Abu Hurayra that the Messenger of Allah, may Allahbless him and grant him peace, said, "Horses are a reward for one man,a protection for another, a burden for another. The one who has themas a reward is the one who dedicates them for use in the way of Allah,and tethers them in a meadow or grassland. Whatever the horse enjoysof the grassland or meadow in the length of its tether are good deedsfor him. If it breaks its tether and goes over a hillock or two, itstracks and droppings are good deeds for him. If it crosses a river anddrinks from it while he did not mean to allow it to drink it, thatcounts as good deeds for him, and the horse is a reward for him. Another man uses his horse to gain self reliance and up-standingness and does not forget Allah's right on their necks andbacks (i.e. he does not ill treat or over-work them). Horses are aprotection for him . Another man uses them out of pride toshow them off and in hostility to the people of Islam. They are aburden on that man." The Messenger of Allah, may Allah blesshim and grant him peace, was asked about donkeys, and he said,"Nothing has been revealed to me about them except this single all-inclusive ayat, 'Whoever does an atom of good will see it, and whoeverdoes an atom of evil, will see it.' " (Sura 99 Ayats 7,8) . Translation Not Available

Book reference: Muwatta Imam Malik Book 21 Hadith 964
Web reference: Muwatta Imam Malik Book 21 Hadith 3