Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: I spent the night with my maternal aunt, Sayyidah Maymunah (may Allah be pleased with her). The Prophet (peace and blessings be upon him) stood up to pray, and he (peace and blessings be upon him) prayed thirteen rak'ahs, among which were the two rak'ahs of Fajr. I estimated his (peace and blessings be upon him) standing in each rak'ah to be as long as "O you wrapped in garments" [Surah al-Muzzammil: 1]. (The narrator) Nuh did not mention the words: "Among them were the two rak'ahs of Fajr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4682