Sahih Muslim , Book 6, Hadith 45 (In-Book) ⓘ
Ibn 'Umar reported: I saw the Messenger of Allah praying (Nafl prayer) on a donkey's back while his face was turned towards Khaibar.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ عَمْرِو بْنِ يَحْيَى الْمَازِنِيِّ، عَنْ سَعِيدِ بْنِ يَسَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي عَلَى حِمَارٍ وَهُوَ مُوَجِّهٌ إِلَى خَيْبَرَ .
also referenced under: Hadith 700 (Reference) & Book 4, Hadith 1501 (USC-MSA)