Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1946
Abu Huraira reported: While the Abyssinians were busy playing with their arms in the presence of the Messenger of Allah (may peace be upon him) 'Umar b. Khattab came there. He bent down to take up pebbles to throw at them (in order to make them go off). The Messenger of Allah (may peace be upon him) said to him: 'Umar, leave them alone.