Sahih Muslim Book 004, Hadith Number 0970.
Chapter : Moderation in the articles of prayers and their shortening and perfection.
Ibn 'Abbas reported: The Messenger of Allah (may peace be upon him) drew aside the curtain (of his apartment) and (he saw) people in rows (saying prayer) behind Aba Bakr. And he said: Nothing remains of the glad tidings of apostle hood, except good visions which a Muslim sees or someone is made to see for him. And see that I have been forbidden to recite the Qur'an in the state of bowing and prostration. So far as Ruk'u is concerned, extol in it the Great and Glorious Lord, and while prostrating yourselves be earnest in supplication, for it is fitting that your supplications should be answered.