What is the ruling if one omits the first Tashahhud and rises up before completing the sitting, specifically if this occurs before attaining full standing (I'tidal)?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If a person omits the first Tashahhud and rises up, and this is remembered before he fully straightens up while standing, he must return to the sitting position to perform the Tashahhud. This is the position held by Alqamah, Ad-Dahhak, Qatadah, Al-Awza'i, Ash-Shafi'i, and Ibn al-Mundhir. The evidence is the narration from Mughirah ibn Shu'bah where the Prophet said: "If one of you stands up in the two rak'ahs and has not yet fully straightened up while standing, let him sit down; but if he has fully straightened up, he should not sit down, and he should perform the two prostrations of forgetfulness." narrated by Abu Dawud and Ibn Majah. Because he omitted an obligation, he must fulfill it, as if his buttocks had not left the ground.
Supporting text
Malik holds that if his buttocks have departed the ground, the action stands. Hassan ibn 'Atiyyah stated that if his knees have moved away from the ground, the action stands. However, the preferred opinion is that he should not sit back down if he remembers before fully straightening up, but if he does sit, it is permissible. Returning is not obligatory if he remembers after beginning a pillar (rukn), such as starting recitation.