From Sahih Bukhari (Hadith 2901):
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 (ﷺ) said, "O `Umar! Allow them (to play)." Ma`mar (the subnarrator) added that they were playing in the Mosque
Reference: Bukhari 2901