Musnad Ahmad 27:1825

Ibn Numayr narrated to us, Ubayd Allah narrated to us, from Nafi’, from Ibn Umar: "The Messenger of Allah (ﷺ) used to recite the prostration (as-Sajdah) outside of prayer, so he would prostrate and we would prostrate with him, until perhaps one of us would not find a place to prostrate in."