عَنْ عُبَادَةَ بْنِ الصَّامِتِ رَضِيَ اللَّهُ عَنْهُ قَالَ: صَلَّى بِنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ صَلَاةَ الْغَدَاةِ فَثَقُلَتْ عَلَيْهِ الْقِرَاءَةُ، فَلَمَّا انْصَرَفَ قَالَ: ((إِنِّي لَأَرَاكُمْ تَقْرَؤُنَ وَرَاءَ إِمَامِكُمْ؟)) قُلْنَا: نَعَمْ وَاللَّهِ يَا رَسُولَ اللَّهِ! إِنَّا لَنَفْعَلُ هَٰذَا، قَالَ: ((فَلَا تَفْعَلُوا إِلَّا بِأُمِّ الْقُرْآنِ فَإِنَّهُ لَا صَلَاةَ لِمَنْ لَمْ يَقْرَأْ بِهَا))
Sayyiduna Ubadah bin Samit (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in the Fajr prayer, and recitation became difficult for him. So, after finishing, he asked: I think you recite behind your Imam? We said: Yes! By Allah, O Messenger of Allah! Without any doubt, we do so. He (peace and blessings be upon him) said: Do not do so (i.e., do not recite behind me) except for Surah Al-Fatihah, because whoever does not recite it, his prayer is not valid.