Surah Al Fatihah is also called Umm-ul-Kitab (the mother of the Book), because it is the first Surah that has been written in the copies of the Quran, and it also the first Surah to be recited in Salat (prayer).
Narrated by Abu Sa’id bin Al-Mu’alla:
While I was offering Salat (prayer) in the mosque, Allah’s Messenger SAW called me but I did not respond to him. Later I said, “O Allah’s Messenger! I was offering Salat (prayer).”
He said, “Didn’t Allah say: ‘Answer Allah (by obeying Him) and (His) Messenger when he (SAW) calls you.’”
He then said to me, “I will teach you a Surah which is the greatest Surah in the Quran, before you leave the mosque.”
Then he got hold of my hand, and when he intended to leave (the mosque), I said to him, “Didn’t you say to me, ‘I will teach you a Surah which is the greatest Surah in the Quran?’”
He said, “Al-hamdu-lillahi Rabbil ‘alamin [all the praises and thanks be to Allah, the Lord of the ‘Alamin (mankind, jinn and all that exists)] (Surah Al-Fatihah) which is As Sab’a Al-Mathani (i.e, the seven repeatedly recited verses) and the Grand Quran which has been given to me.”
[Sahih Al Bukhari, 6/4474 (O.P.1)]