Hadith Q489
بِلَفْظٍ ادَّعَتْ فِرْقَةٌ أَنَّهَا دَالَّةٌ عَلَى أَنَّ تَرْكَ قِرَاءَةِ فَاتِحَةِ الْكِتَابِ يَنْقُصُ صَلَاةَ الْمُصَلِّي لَا تُبْطِلُ صَلَاتَهُ وَلَا يَجِبُ عَلَيْهِ إِعَادَتُهَا.
On this basis, one sect has claimed that if a person omits the recitation of Surah Al-Fatihah, the prayer becomes deficient but is not invalid, nor is it obligatory for him to repeat that prayer.
Hadith 489
Hazrat Abu Saib (may Allah have mercy on him) narrates that he heard Sayyiduna Abu Hurairah (may Allah be pleased with him) saying that the Messenger of Allah (peace and blessings be upon him) said: "Whoever offered a prayer and did not recite Umm al-Qur'an (Surah al-Fatihah) in it, then that prayer is incomplete, that prayer is incomplete, that prayer is incomplete, it is not complete." So I submitted, "O Abu Hurairah (may Allah be pleased with him), sometimes I am behind the Imam (so how should I recite)?" He said, so he pressed my arm and said, "O Farsi, (at that time) you should recite it in your heart."