Musnad Ahmad 30:961

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib, from Abu Hurayrah, who said: While the Abyssinians were playing with their spears in the presence of the Messenger of Allah, may Allah bless him and grant him peace, Umar entered and reached for pebbles to pelt them with. The Prophet, may Allah bless him and grant him peace, said to him: "Leave them, O Umar."