أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ عَمْرِو بْنِ يَحْيَى، عَنْ سَعِيدِ بْنِ يَسَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي عَلَى حِمَارٍ وَهُوَ مُتَوَجِّهٌ إِلَى خَيْبَرَ ‏.‏

It was narrated that Ibn 'Umar said: "I saw the Messenger of Allah (PBUH) praying on a donkey, when he was heading toward Khaibar." Translation Not Available

Book Ref: Sunan An Nisai Book 8 Hadith 740
Web Ref:  Sunan An Nisai Vol 1 Book 8 Hadith 741