Bulugh al-Maram #422

Narrated '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." . 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).