What is the ruling on praying behind an imam who does not sit between the two prostrations due to his old age, even though he sits for the Tashahhud?

If the Imam raises his thighs between the two prostrations and does not sit on his posterior, it is not sufficient unless there is an excuse. However, if he sits muq'iyan, that is permissible. Al-iq'a' is to keep the feet upright and sit on them with the buttocks. Al-Nawawi mentioned in Al-Majmoo' that all hadiths prohibiting al-iq'a' are weak. Whereas it was reported from Ibn Abbas, Ibn Omar, Ibn Al-Zubair, and Tawus that al-iq'a' between the two prostrations is Sunnah, which is to place the tips of one's toes on the ground and place one's buttocks on one's heels, with one's knees on the ground. As for the prohibited iq'a', it is to place one's buttocks on the ground, raise one's shins, and place one's hands on the ground, or to sit with one's thighs raised, similar to the iq'a' of a dog or a wild beast.