It was narrated that Ali bin Abi Talib, may Allah be pleased with him, said: "A mule was given as a gift to the Messenger of Allah (ﷺ) and he rode it." 'Ali said: "If we mate a donkey with a horse, we will have one like this." The Messenger of Allah (ﷺ) said: "That is only done by those who do not know."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، عَنْ أَبِي الْخَيْرِ، عَنِ ابْنِ زُرَيْرٍ، عَنْ عَلِيِّ بْنِ أَبِي طَالِبٍ، رضى الله عنه قَالَ أُهْدِيَتْ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم بَغْلَةٌ فَرَكِبَهَا فَقَالَ عَلِيٌّ لَوْ حَمَلْنَا الْحَمِيرَ عَلَى الْخَيْلِ لَكَانَتْ لَنَا مِثْلَ هَذِهِ . قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّمَا يَفْعَلُ ذَلِكَ الَّذِينَ لاَ يَعْلَمُونَ " .
sunnah.com