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." [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).

وَعَنْ عُمَرَ ‏- رضى الله عنه ‏- قَالَ : { يَا أَيُّهَا اَلنَّاسُ إِنَّا نَمُرُّ بِالسُّجُودِ فَمَنْ سَجَدَ فَقَدْ أَصَابَ , وَمَنْ لَمْ يَسْجُدْ فَلَا إِثْمَ عَلَيْهِ } .‏ رَوَاهُ اَلْبُخَارِيُّ .‏ وَفِيهِ : { إِنَّ اَللَّهَ] تَعَالَى [ لَمْ يَفْرِضْ اَلسُّجُودَ إِلَّا أَنْ نَشَاءَ } .‏ [1]‏ وَهُوَ فِي " اَلْمُوَطَّأِ.‏ [2]‏