(11) باب وُجُوبِ قِرَاءَةِ الْفَاتِحَةِ فِي كُلِّ رَكْعَةٍ وَإِنَّهُ إِذَا لَمْ يُحْسِنِ الْفَاتِحَةَ وَلاَ أَمْكَنَهُ تَعَلُّمُهَا قَرَأَ مَا تَيَسَّرَ لَهُ مِنْ غَيْرِهَا
Mahmud b. al-Rabi', on whose face the Messenger of Allah (ﷺ) squirted water from the well, reported on the authority of 'Ubada b. as- Samit that the Messenger of Allah (ﷺ) said:
He who does not recite Umm al-Qur'an is not credited with having observed prayer.
Reference: Sahih Muslim 394 c
In-book reference: Book 4, Hadith 39
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 773