O you who have believed, respond to Allah and to the Messenger when he calls you to that which gives you life. And know that Allah intervenes between a man and his heart and that to Him you will be gathered.
Narrated Abu Sa`id bin Al-Mu'alla:
While I was praying in the Mosque, Allah's Messenger (sallallaahu alayhi wa sallam) called me but I did not respond to him.
Later I said, "O Allah's Messenger (sallallaahu alayhi wa sallam)! I was praying."
He said, "Didn't Allah say:
"Give your response to Allah (by obeying Him) and to His Apostle when he calls you."
He then said to me, "I will teach you a Surah which is the greatest Surah in the Qur'an, 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 Qur'an?'
He said, "Al-Hamdu-Li l-lah Rabbi-l-`alamin (i.e. Praise be to Allah, the Lord of the worlds) which is Al-Sab' Al-Mathaani (i.e. seven repeatedly recited Verses) and the Grand Qur'an which has been given to me."
[Sahih Al-Bukhaari Hadith No. 4474]
Soorah Al-Faatiha is the greatest Soorah in the Qur'aan. And it is also called Al-Sab' Al-Mathaani [which means the seven repeatedly recited Verses].