Sahih Muslim Book 7, Hadith Number 2978.

Chapter : Excellence of flinging pebbles at Jamrat al-Aqaba on the day of sacrifice (10th of Dhu’l-Hijja) while riding.

Umm al-Husain (Allah be pleased with her) reported: I performed Hajj along with Allah’s Messenger (may peace be upon him) on the occasion of the Farewell Pilgrimage and saw Usama and Bilal (too), one of whom had caught hold of the lose string of the she-camel of Allah’s Apostle (may peace be upon him) while the other one was raising his cloth (over his head) protecting him from the heat, till he flung pebbles at Jamrat al-‘Aqaba.