What is the ruling regarding the intent (Niyyah) when saying the initial Takbir while joining during Ruku?

Primary text

If the follower intends only the opening Takbir (Takbirat al-Ihram), it is sufficient. If the follower intends both the opening Takbir and the Takbir for Ruku simultaneously, the apparent ruling from Ahmad is that it is invalid because it mixes the obligatory (Ihram) with the non-obligatory (Ruku's Takbir) in one intention. This is likened to saying 'Rabbana wa lakal hamd' upon rising from Ruku while intending it as a mere utterance rather than as Dhikr. However, other narrations from Ahmad state that whether the follower intends the opening Takbir or not, the prayer is valid, since the person clearly intended to pray. The intention for Ruku does not invalidate the intention for Ihram.