Sahih Bukhari Volume 4, Book 52, Hadith Number 150.

Narated By 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.