Hadith Q490
إِذِ النَّقْصُ فِي الصَّلَاةِ يَكُونُ نَقْصَيْنِ، أَحَدُهُمَا لَا تُجْزِئُ الصَّلَاةُ مَعَ ذَلِكَ النَّقْصِ، وَالْآخَرُ تَكُونُ الصَّلَاةُ جَائِزَةً مَعَ ذَلِكَ النَّقْصِ لَا يَجِبُ إِعَادَتُهَا، وَلَيْسَ هَذَا النَّقْصُ مِمَّا يُوجِبُ سَجْدَتَيِ السَّهْوِ مَعَ جَوَازِ الصَّلَاةِ.
Because there are two types of deficiencies in prayer: one deficiency is such that, if present, the prayer is not sufficient; the other deficiency is such that, with it, the prayer becomes valid, and repeating it is not obligatory, and this is not the deficiency that, along with the prayer being valid, makes two prostrations of forgetfulness obligatory.
Hadith 490
Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace be upon him) said: "Any prayer in which Surah Al-Fatihah is not recited is not sufficient." (Abdur Rahman) says: I asked, "If I am praying behind the Imam?" So he (the Prophet) took my hand and said, "O Persian, (at that time) recite it in your heart."