It is that when the Messenger of Allah (peace be upon him) calls people to matters that give life, it is obligatory for them to respond to his call, even if they are performing prayer.

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.