The Messenger of Allah ﷺ said: He who said his prayer, but did not recite the opening chapter of al-Kitab, his prayer is incomplete. He repeated it thrice.
سَمِعْتُ مِنْ أَبِي وَمِنْ أَبِي السَّائِبِ وَكَانَا جَلِيسَىْ أَبِي هُرَيْرَةَ قَالاَ قَالَ أَبُو هُرَيْرَةَ قَالَ رَسُولُ اللَّهِ ﷺ مَنْ صَلَّى صَلاَةً لَمْ يَقْرَأْ فِيهَا بِفَاتِحَةِ الْكِتَابِ فَهْىَ خِدَاجٌ يَقُولُهَا ثَلاَثًا بِمِثْلِ حَدِيثِهِمْ