Musannaf Ibn Abi Shayba 3:85

Waki’ narrated to us, from Sufyan, from Ibn Abi Layla, from al-Hakam, who said: If he says "Subhan Allah" (glorifying Allah) or "La ilaha illa Allah" (declaring the oneness of Allah) at the opening of the prayer, it suffices him in place of the Takbir (saying Allahu Akbar).