Musnad Ahmad 30:3184

I read to Abd al-Rahman: Malik, from Abdullah ibn Yazid, the freed slave of al-Aswad ibn Sufyan, from Abu Salamah ibn Abd al-Rahman, that Abu Hurairah recited to them: "When the sky has split [Idha al-sama'u inshaqqat]," and he prostrated during it. When he finished, he informed them that the Messenger of Allah (peace and blessings of Allah be upon him) prostrated during it.