Sahih Muslim Book 7, Hadith Number 2919.

Chapter : It is permissible to observe Tawaf on the back of a camel or any other riding beast, and to touch the black stone with a stick.

jabir b. ‘Abdullah (Allah be pleased with them) reported that Allah’s Apostle (may peace be upon him) circumambulated the House (and ran) between al-Safa and al-Marwa on the back of his she-camel, at the occasion of the Farewell Pilgrimage, so that the people should see him and he should be conspicuous, and they should be able to ask him (questions pertaining to religion), and the people had crowded round him. In the Hadith transmitted on the authority of Ibn Khashram no mention Is made of: “So that they should ask him.”