What is the manner of sitting for the Tashahhud in four-rak'ah, three-rak'ah, and two-rak'ah prayers, and does the ruling differ for the Tashahhud in Fajr prayer?

The jurists differed on the manner of sitting for the tashahhud. The most accurate opinions state that tawarruk is prescribed for the final tashahhud of a four-unit and three-unit prayer, and iftirash for a two-unit prayer (whether obligatory or supererogatory) and for the middle tashahhud. Iftirash is sitting on the left foot while keeping the right foot upright. Tawarruk is extending the left foot, keeping the right foot upright, and bringing the left foot out from under the worshipper's right side, resting the buttocks on the ground.

The proof for this is the hadith of Abu Humayd al-Sa'idi, describing the prayer of the Prophet, peace and blessings be upon him: "When he was in the two rak'ahs, he would sit on his left foot and keep the right foot upright. When he sat in the last rak'ah, he would bring forward his left foot and keep the other upright and sit on his buttocks" (narrated by Al-Bukhari). In another narration: "Until, when it was the rak'ah in which the prayer would conclude, he would extend his left foot and sit on his side, performing tawarruk, then he would give the salutation." This is evidence for tawarruk in the final tashahhud. The evidence for iftirash is the hadith of Wa'il ibn Hujr that the Prophet, peace and blessings be upon him, "spread his left foot."