Sahih Bukhari Volume 6, Book 61, Hadith Number 528.

Narrated By Abu Sa’id Al-Mu’alla : While I was praying, the Prophet called me but I did not respond to his call. Later I said, “O Allah’s Apostle! I was praying.” He said, “Didn’t Allah say: ‘O you who believe! Give your response to Allah (by obeying Him) and to His Apostle when he calls you’?” (8.24)

He then said, “Shall I not teach you the most superior Surah in the Qur’an?” He said, ‘(It is), ‘Praise be to Allah, the Lord of the worlds. ‘ (i.e., Surat Al-Fatiha) which consists of seven repeatedly recited Verses and the Magnificent Qur’an which was given to me.”