His saying: "When the heaven splits asunder," means: it is cleft, and that will be on the Day of Resurrection.
قَوْله " ﴿إِذا السَّمَاء انفطرت﴾ يَعْنِي: انْشَقَّتْ، وَذَلِكَ يَوْمَ الْقِيَامَةِ