'Umar (RA):
He said, "O people! We pass (in our recitation) verses to be prostrated at, so whoever prostrates has done the right thing, and there is no sin upon the one who does not prostrate."
[al-Bukhari reported it].
It also contains the words: 'Allah the Most High has not made the prostration (of recitation) compulsory, unless one wishes (to do so).'
[This Hadith is in the Muwatta' of Imam Malik).