Sa'id bin Jubayr narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning 'al-Sab' al-Mathani' (the seven oft-repeated verses) that it refers to Surah al-Fatihah. Ibn Abbas (may Allah be pleased with them both) recited it, counting 'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1] with it, as seven (verses).
Ibn Jurayj (may Allah have mercy on him) said: I asked my father: Did Sa'id bin Jubayr tell you, on the authority of Ibn Abbas (may Allah be pleased with them both), that 'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1] is a verse of the Book of Allah? He replied: Yes! And Ibn Abbas (may Allah be pleased with them both) recited 'In the name of Allah, the Most Gracious, the Most Merciful' [Surah al-Fatihah: 1] in both rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2399