Sahih al-Bukhari Book 77, Hadith 182
The worst of three (persons riding one, animal) was mentioned in `Ikrima's presence `Ikrima said,
"Ibn `Abbas said, '(In the year of the conquest of Mecca) the Prophet (ﷺ) came and mounted Qutham
in front of him and Al-Fadl behind him, or Qutham behind him and Al-Fadl in front of him.' Now
which of them was the worst off and which was the best?"
When the Prophet (ﷺ) arrived at Mecca, the children of Bani `Abdul Muttalib received him. He then mounted one of them in front of him and the other behind him.
While I was riding behind the Prophet (ﷺ) and between me and him and between me and him there was only the back of the saddle, he said, "0 Mu`adh!" I replied, "Labbaik, 0 Allah's Messenger (ﷺ), and Sa`daik!" he said, "Do you know what is Allah's right upon his slave?" I said, "Allah and His Apostle know best" He said "Allah's right upon his slaves is that they should worship Him alone and not worship anything else besides Him." Then he proceeded for a while and then said, "O Mu`adh bin Jabal!" I replied, "Labbaik, O Allah's Messenger (ﷺ):, Sa`daik!' He said, "Do you know what is the right of the slaves upon Allah if they do that?" I replied, "Allah and His Apostle know best." He said, "The right of the slaves upon Allah is that He will not punish them (if they do that).