Sahih Muslim Book 4, Hadith Number 1189.

Chapter : Forgetfulness in prayer and prostration as compensation for it.

Ibn ‘Umar reported: The Messenger of Allah (may peace be upon him) while reciting the Qur’an recited its scarab containing sajda, and he performed prostration and we also prostrated along with him (but we were so overcrowded) that some of us could not find a place for our forehead (when prostrating ourselves).