Musannaf Ibn Abi Shayba 3:2019

Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, that he heard Abd al-Rahman al-A'raj saying: Abu Hurayrah used to prostrate during "When the sky has split asunder" (Surah Al-Inshiqaq). If it was recited and he was behind the Imam, and the Imam did not prostrate, then Abu Hurayrah would gesture with his head.