Musnad Ahmad 968:1

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Yazid ibn Abi Ziyad, from Sulayman ibn ‘Amr ibn al-Ahwas, from his mother, that she witnessed the Prophet, may Allah bless him and grant him peace, at Jamrat al-‘Aqabah while the people were throwing [pebbles], and he said: "O people, do not kill, or do not destroy, yourselves. And throw at the Jamrah, or the Jamarat, with [pebbles] like the size of the pebbles of al-khadhf [throwing a pebble between the thumb and index finger]." And Shu‘bah gestured with the tip of his index finger.