Is the prayer of one who begins the takbirat al-ihram while grasping his right hand over his left valid?

Raising the hands during the takbīrat al-iḥrām (the opening Allahu Akbar) is a Sunnah. If one says the takbīrat al-iḥrām while keeping their hands clasped, they have omitted that Sunnah. Their prayer is valid, but one should be keen to adhere to the Sunan, for their reward is great.