Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 150
Narrated Abu Huraira:
While some Ethiopians were playing in the presence of the Prophet, 'Umar came in, picked up a stone and hit them with it. On that the Prophet said, "O 'Umar! Allow them (to play)." Ma'mar (the sub-narrator) added that they were playing in the Mosque.