Is it obligatory to place the hands on the chest after the opening takbeer when joining the Imam in rukoo‘, or is it permissible to go directly into rukoo‘?

Whoever catches the Imam while he is bowing must say the Takbirat al-Ihram (opening Takbir) while standing, then immediately go down into the bowing position. It is not prescribed for him to place his hands on his chest after the Takbirat al-Ihram.