Abu Saeed bin Mualla narrates: I was performing prayer in the mosque when the Prophet Muhammad (peace be upon him) called me, but I did not go to him. (When I later presented myself before him), I said: O Messenger of Allah! I was performing prayer. The Prophet Muhammad (peace be upon him) said: Did Allah Almighty not say: "When Allah and His Messenger call you, then you should go to them." Then the Prophet Muhammad (peace be upon him) said: Shall I not teach you a Surah which is the greatest Surah in the Quran? I said: Yes! The Prophet Muhammad (peace be upon him) said: "Alhamdulillahi Rabbil Alameen"—this is the Sab‘ al-Mathani and it is the Quran which has been given to me.
(Imam Ibn Hibban, may Allah have mercy on him, says:) The statement of the Prophet Muhammad (peace be upon him), "This is the greatest Surah," means in terms of reward (it is the greatest). It does not mean that one part of the Quran is superior to another. The name of Sayyiduna Abu Saeed bin Mualla (may Allah be pleased with him) is Rafi‘ bin Mualla bin Lawdhan bin Haritha. He passed away in the year 74 Hijri.
Hadith Referenceصحیح ابن حبان / كتاب الرقائق / 777
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (1311): خ.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط البخاري، رجاله رجال الشيخين غير مسدد فمن رجال البخاري