Thanks to Faddy.
Imam al-Shafi’i said in his book, Al-Umm (1/214), “The takbir is how the Messenger of Allah, peace and blessings be upon him, used to make takbir in the prayer: Allahu Akbar. Then, the Imam begins to say, Allahu Akbar, Allahu Akbar, Allahu Akbar, until he has said this three times, and if he says more, it is good. And if he wants to add to it, he says, Allahu Akbar kabiran, wa al-hamdu lillahi kathiran, wa subhan Allahi bukratan wa asilan, Allahu Akbar, wa la na`budu illa iyyah, mukhlisina lahu al-dina wa law kariha al-kafiran, la ilaha illa Allahu wahdah, sadaqa wa`dah, wa nasara `abdah, wa hazama al-ahzaba wahdah, la ilaha illa Allahu, wa Allahu Akbar. This is excellent. Whatever dhikr he wishes to add to this, I have loved it, though I love that he begin with three takbirs successively, and if he limits himself to only one, I have permitted it. “