بُلُوغُ الْمَرَامِ
Bulugh al-Maram 346
The Book of Prayer
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]
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).
Sunnah.com reference: Book 2, Hadith 247