He must say "Allahu Akbar" while standing for prayer, then bow; it is not permissible for him to bow directly without saying "Allahu Akbar". Therefore, if he says the takbir while he is closer to standing than to bowing, his initiation of the prayer is valid, even if he intended it as the takbir for bowing, and his intention is invalidated; because the imam's position in bowing does not require saying the takbir twice. However, if he is closer to bowing than to standing, his initiation is not valid; because it is a condition for the validity of the opening takbir that it be said while standing or slightly bent before he is closer to bowing, and here the condition is not met. See: Maraqi al-Falah, p. 217.