It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) or Sayyiduna Abdullah bin Mughaffal (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Beware of khadhf (flicking pebbles with the fingers), for it breaks teeth and gouges out eyes, and it does not wound the enemy."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15657