Sahih Bukhari Volume 2, Book 19, Hadith Number 183.
Narated By Rabi’a : ‘Umar bin Al-Khattab recited Surat-an-Nahl on a Friday on the pulpit and when he reached the verse of Sajda he got down from the pulpit and prostrated and the people also prostrated. The next Friday ‘Umar bin Al-Khattab recited the same Sura and when he reached the verse of Sajda he said, “O people! When we recite the verses of Sajda (during the sermon) whoever prostrates does the right thing, yet it is no sin for the one who does not prostrate.” And ‘Umar did not prostrate (that day). Added Ibn ‘Umar “Allah has not made the prostration of recitation compulsory but if we wish we can do it.”