Musnad Abu Ya'la al-Mawsili 160:39

Abu Ma'mar Isma'il ibn Ibrahim narrated to us, Hushaym narrated to us, 'Ali ibn Zayd informed us, from Abu Rafi', who said: I prayed the final 'Isha' prayer behind Abu Hurayrah, and we prostrated during "When the sky has split [Idha al-sama'u inshaqqat]." When he finished his prayer, I said, "Do you prostrate during 'When the sky has split'?" He said, "The Messenger of Allah (may Allah bless him and grant him peace) prayed with us and prostrated during it, so I shall not cease to prostrate during it."