What is the ruling on iq'a (sitting on one's heels) in prayer between the two prostrations, during the tashahhud, and during the final tashahhud in the Dhuhr, Asr, and Isha prayers?

Iq'a' (placing the feet upright and sitting on the heels) is permissible only between the two prostrations according to the Shafi'i school, and not throughout the entire prayer. It is authentically reported from Ibn Abbas that iq'a' is a Sunnah (definition), and Muslim narrated it. The prayer of one who performs iq'a' in all their sittings is valid, but they have acted contrary to the Sunnah, as mentioned by An-Nawawi. Iftirash (sitting on the left foot, with the right foot upright) is better than iq'a', because it was the predominant sitting posture of the Prophet, peace be upon him, and because its narrations are more numerous.