It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Beware of making the backs of your animals into pulpits, for Allah Almighty says that He has subjected them to you so that they 'carry your loads to a land you could not reach except with great hardship to yourselves' [Surah al-Nahl: 7], and Allah has made the earth for you, so fulfil your needs upon it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10335