Sahih Muslim Book 4, Hadith Number 777.

Chapter : The reciting of Takbir at the time of bowing and rising in prayer except rising after Ruku’ when it is said: Allah listened to him who praised Him.

Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: He who said his prayer, but did not recite the opening chapter of al-Kitab, his prayer is incomplete. He repeated it thrice.