Musannaf Ibn Abi Shayba 3:90

Narrated to us Abd al-A’la, from Ma’mar, from al-Zuhri, that he said regarding a man if he forgets when he says the takbir to open the prayer: He should say the takbir when he remembers. If he does not remember until he prays, his prayer is valid, and the takbir of the bowing (ruku’) suffices him.