It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) passed by Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) while he was praying, so the Prophet (peace and blessings of Allah be upon him) called him. He turned towards the Prophet (peace and blessings of Allah be upon him) (and continued his prayer), then (after completing the prayer) he came to the service of the Messenger of Allah (peace and blessings of Allah be upon him) and said, "O Messenger of Allah, «‏‏‏‏السَّلامُ عَلَیْکَ» ‏‏‏‏." The Prophet (peace and blessings of Allah be upon him) said: " «‏‏‏‏وَعَلَيْك السَّلام» ‏‏‏‏" "O Ubayy, when I called you, what prevented you from coming?" He replied, "O Messenger of Allah, I was praying." The Prophet (peace and blessings of Allah be upon him) said: "Did you not find this command in the Book of Allah" «‏‏‏‏يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ» ‏‏‏‏ "O you who believe, obey Allah and His Messenger when He calls you to that which gives you life." He replied, "Why not (this command is certainly present), may my parents be sacrificed for you." Sayyiduna Ubayy says, "InshaAllah, I will not be negligent in this in the future." This is the hadith of Ibn Wahb.

Sayyiduna Abu Sa’id bin Mu’alla (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) passed by me while I was (praying) in the mosque. The Prophet (peace be upon him) called me, but I did not come (and continued my prayer). Then when I came to the Prophet (peace be upon him), he said:“Why did you not come to me?”I replied,“I was praying.”The Prophet (peace be upon him) said:“Did Allah not say” «‏‏‏‏يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ» ‏‏‏‏ [ سورة الأنفال: 24 ] “O you who believe! Respond to Allah and to the Messenger when he calls you to that which gives you life.”Then he said:“Shall I not teach you the greatest Surah of the Qur’an before you leave the mosque?”So when the Prophet (peace be upon him) was about to leave the mosque, I reminded him. The Prophet (peace be upon him) said:“ «‏‏‏‏الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ» ‏‏‏‏ that is (Surah Al-Fatihah), the seven oft-repeated verses (As-Sab’ul Mathani), and this is the Great Qur’an which has been given to me.”

Sayyiduna Abu Sa‘id bin Ma‘la (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) passed by me while I was praying, so he (peace be upon him) called me. He narrated the hadith similar to the one mentioned above, but in this narration, instead of "the most virtuous surah," the words "the greatest surah" are used.

If a person performing prayer, due to ignorance or lack of awareness, utters the same words during the prayer, it will not invalidate the prayer.

Abu Hurairah (may Allah be pleased with him) narrates that when the Messenger of Allah (peace be upon him) established the prayer, we also stood with him. Then a Bedouin man supplicated in the prayer like this: "O Allah, have mercy on me and on Muhammad (peace be upon him), and do not have mercy on anyone else along with us." When the Messenger of Allah (peace be upon him) finished the prayer with salam, he said to the Bedouin: "You have made Allah’s vast mercy narrow."

So his prayer will not be invalidated, nor will it be necessary to repeat that prayer. Although it is permissible to take evidence from the narration of Qaboos bin Abi Zubyan, my heart is not satisfied with it.

Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that the Prophet Muhammad (peace be upon him) led the prayer in Mina (and he forgot and) a word slipped from his tongue. The hypocrites heard it and made much talk about it. They began to say that the Prophet (peace be upon him) has two hearts; did you not hear his words and speech in the prayer? One of his hearts is with you and the other heart is with his companions. Then these verses were revealed: «‏‏‏‏يَا أَيُّهَا النَّبِيُّ اتَّقِ اللَّهَ وَلَا تُطِعِ الْكَافِرِينَ وَالْمُنَافِقِينَ ۗ ....‏ مَّا جَعَلَ اللَّهُ لِرَجُلٍ مِّن قَلْبَيْنِ فِي جَوْفِهِ» ‏‏‏‏ [ سورة الأحزاب: 1-4 ] "O Prophet, fear Allah and do not obey the disbelievers and the hypocrites. Indeed, Allah is All-Knowing, All-Wise. And follow that which is revealed to you from your Lord. Indeed, Allah is well aware of what you do. And put your trust in Allah, and Allah is sufficient as a disposer of affairs. Allah has not placed two hearts in any man's chest."