Sunan an-Nasa'i, Volume 1, Book 8, Hadith 741 (DarusSalam) ⓘ
It was narrated that Ibn 'Umar said: "I saw the Messenger of Allah praying on a donkey, when he was heading toward Khaibar."
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 .
also referenced under: Hadith 740 (Reference) & Book 8, Hadith 53 (In-Book)