Sayyiduna Wabisah bin Ma'bad (may Allah be pleased with him) narrates: I heard the Prophet (peace and blessings be upon him) say: "Do not make the backs of animals into pulpits (that is, do not stay mounted on them while they are standing still)." I heard the Prophet (peace and blessings be upon him) say: "The worst of animals is the fox."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Mubashshir bin Ubayd, who is da'if (weak).