Musnad Ahmad 282:3

Rawh narrated to us, he said: Shu'bah narrated to us, from Yazid ibn Abi Ziyad, from Sulayman ibn 'Amr ibn al-Ahwas al-Azdi, from his mother, from the Prophet, may Allah bless him and grant him peace, that she heard him saying at Jamrat al-'Aqabah: "O people, do not kill yourselves, and throw the Jamrah—or the Jamarat—with pebbles the size of those used for throwing (al-khadhf)."