Answered by Shaykh Faraz Rabbani
If one reads his salah in the wrong order e.g. begins to read fatihah in tashahud (due to forgetfulness or distraction) but then corrects it, does the salah become invalid?
In the Name of Allah, the Beneficent, the Merciful, and all blessings and peace upon our master Muhammad, his Folk, Companions and those who follow their noble way,
It is wajib to maintain the order between the fard and wajib actions of the prayer. It is also wajib not to delay any of the actions of the prayer without reason beyond the extent of 3 tasbihs (=saying Subhan Allah).
If a wajib is left unintentionally, it is wajib to do forgetfulness prostrations at the end of the prayer. If omitted, the prayer must be repeated.
If a wajib is left intentionally, it is sinful. Forgetfulness prostrations cannot be made for this. Rather, one would have to: a) repent; b) repeat the prayer. The prayer is repeated in order to fix the gross shortcoming in it, not because it was invalid. (Leaving conditions and integrals, all fard, invalidates the prayer. Leaving a wajib makes the prayer valid but unsound, requiring one of the two mentioned correctors)
And Allah knows best.
Wassalam,
Faraz Rabbani.