Musnad Abu Ya'la al-Mawsili 7:21

Abu Bakr ibn Abi Shaybah narrated to us, Bakr ibn Abd al-Rahman narrated to us, from Ibn Abi Layla, from Humayd ibn Abi Abd Allah, from Abu Salamah, from his father, from the Prophet (peace and blessings of Allah be upon him), who said: "I saw him prostrate during 'When the sky has split [Idha al-sama'u inshaqqat]' (Surah Al-Inshiqaq) ten times."