When the Prophet (ﷺ) recited Surat As-Sajda and we were with him, he would prostrate and we also would prostrate with him and some of us (because of the heavy rush) would not find a place (for our foreheads) to prostrate on.
Sahih al-Bukhari Book 17, Hadith 10
← Previous Hadith When the Prophet (ﷺ) recited a Sura that contained the prostration he would prostrate and we would do the same and some of us (because of the heavy rush) could not find a place for prostration.

Sahih al-Bukhari Book 17, Hadith 9 Next Hadith → `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."

Sahih al-Bukhari Book 17, Hadith 11