It is narrated from Rabi'ah bin Abdullah (may Allah have mercy on him) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) recited Surah al-Nahl on Friday (on the pulpit). When the verse of prostration, "And to Allah prostrates whatever is in the heavens and whatever is on the earth" [Surah al-Nahl: 49], came, Sayyiduna Umar (may Allah be pleased with him) came down from the pulpit and prostrated, and the people prostrated too. On the following Friday he recited the same surah again, and when he reached the verse of prostration he said: O people, we have not been commanded to prostrate; so whoever prostrates has done well and has done what is right, and whoever does not prostrate, there is no sin upon him. And Sayyiduna Umar (may Allah be pleased with him) did not prostrate. Nafi' (may Allah have mercy on him) added: Your Lord has not made the prostration of recitation obligatory upon us, but has left it to our choice.
(b) This is through the chain of Imam al-Bukhari (may Allah have mercy on him). At its end is this addition: Nafi' (may Allah have mercy on him) narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that Allah has not made the prostrations of recitation obligatory upon us, except that if we wish, we may perform them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3755