Mishkat al-Masabih 29 Excellent Qualities and Description of the Prophet كتاب الفضائل والشمائل 81

وَعَنْ أَنَسٍ يُحَدِّثَ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ كانَ يعودُ المريضَ وَيتبع الْجِنَازَة ويجيب دَعْوَة الْمَمْلُوك ويركب الْحمار لَقَدْ رَأَيْتُهُ يَوْمَ خَيْبَرَ عَلَى حِمَارٍ خِطَامُهُ لِيفٌ. رَوَاهُ ابْنُ مَاجَهْ وَالْبَيْهَقِيُّ فِي «شُعَبِ الْإِيمَانِ»

Anas used to tell of the Prophet that he would visit the sick, attend funerals, accept a slave’s invitation, and ride on a donkey. He said he had seen him on a donkey at the battle for Khaibar using a rein of palm-fibres. Ibn Majah and Baihaqi, in Shu’ab al Iman , transmitted it.