What is the ruling on praying behind an Imam who does not raise his hands during the opening Takbir (Takbirat al-Ihram) and lets his hands hang down during prayer?

It is sunnah (recommended) to raise the hands during the takbirat al-ihram (the opening takbir of prayer), by the consensus of jurists, due to the Prophet's (peace be upon him) practice. This is the preferred action for one praying, whether as an imam (prayer leader) or a ma'mum (follower). Whoever does not do so has deviated from the Sunnah, but their prayer is valid. As for letting the hands hang down ( isbal ) in prayer, the Malikis held this view, while the majority of scholars went with the recommendation of placing the right hand over the left during the standing ( qiyam ) posture, citing the Prophet's (peace be upon him) practice as their evidence. There is no impediment to praying behind an imam who omits these two recommended actions, but he should be gently reminded of what is better.