What is the ruling on beginning the final Tashahhud when rising from prostration and before sitting for the moderate posture?

The final Tashahhud and the sitting for it are pillars of prayer according to the Hanbalis and Shafi'is. Beginning it before sitting invalidates the prayer according to those who consider it a pillar. However, some scholars view leniency in this matter due to the difficulty of avoiding it and the frequent forgetfulness concerning it.