The follower’s prayer when he bows as the imām is rising from bowing

Answer: The scholars of the school said: if the imām precedes the follower by two successive, substantive pillars, the follower’s prayer is invalid. Their statement accords with the famous ḥadīth: “The imām is only appointed to be followed … so when he bows, bow …” and so on. If the imām raises his head from bowing before the follower has bowed, then the follower has failed to do what that ḥadīth commands—namely, following. However, the follower’s prayer may still be valid—that is, if he bows before the imām becomes fully upright from bowing. And Allah knows best. All praise is for Allah, Lord of the worlds, and may blessings and peace be upon our master Muḥammad and his pure family.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1