Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: I was riding behind the Prophet (peace and blessings be upon him) on a donkey called Ya'fur, and I began to sweat, so he (peace and blessings be upon him) ordered me to take a bath (ghusl).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12594