Sunan an-Nasa'i 741
It was narrated from Anas bin Malik that he saw the Messenger of Allah (ﷺ) praying on a donkey while he was riding, praying toward Khaibar with the Qiblah behind him. 

   Abu 'Abdur-Rahman (An-Nasa'i) said:
We do not know of anyone who reported anything to support what 'Amr bin Yahya said about praying on a donkey. As for the Hadith of Yahya bin Sa'eed from Anas, what is correct is that it is Mawquf. [1] And Allah knows best.

[1] That is a saying or action of a Companion of the Prophet (ﷺ)
أَخْبَرَنَا مُحَمَّدُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ بْنُ عُمَرَ، قَالَ حَدَّثَنَا دَاوُدُ بْنُ قَيْسٍ، عَنْ مُحَمَّدِ بْنِ عَجْلاَنَ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّهُ رَأَى رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي عَلَى حِمَارٍ وَهُوَ رَاكِبٌ إِلَى خَيْبَرَ وَالْقِبْلَةُ خَلْفَهُ ‏.‏ قَالَ أَبُو عَبْدِ الرَّحْمَنِ لاَ نَعْلَمُ أَحَدًا تَابَعَ عَمْرَو بْنَ يَحْيَى عَلَى قَوْلِهِ يُصَلِّي عَلَى حِمَارٍ وَحَدِيثُ يَحْيَى بْنِ سَعِيدٍ عَنْ أَنَسٍ الصَّوَابُ مَوْقُوفٌ وَاللَّهُ سُبْحَانَهُ وَتَعَالَى أَعْلَمُ ‏.‏