Musnad Ahmad 1153:1806

Isma'il narrated to us, he said: Khalid al-Hadhdha' informed us, from Muhammad ibn Sirin, he said: Aisha said: "The standing of the Messenger of Allah, peace and blessings of Allah be upon him, in the two rak'ahs before the dawn prayer was [for] the duration of what one recites the Opening of the Book [Al-Fatiha]."