Musannaf Ibn Abi Shayba 3:1861

Mu'adh ibn Mu'adh narrated to us, from 'Ali ibn Suwayd ibn Manjuf, who said: Abu Rafi' al-Sa'igh informed me, saying: 'Umar led us in the final 'Isha' prayer. He recited in one of the first two rak'ahs: "When the sky has split [Idha al-sama'u inshaqqat]," and he prostrated, and we prostrated with him.