Is it impermissible to limit oneself to a single form of tashahhud in prayer, based on Sheikh Al-Albani's statement that combining different forms of tashahhud and salat is an innovation, and that the Sunnah is to recite this sometimes and that sometimes?

A Muslim is permitted to restrict himself to any of the established forms of the tashahhud narrated from the Prophet, peace and blessings be upon him. There is no harm in reciting the tashahhud with one form at one time and another form at another time, even if it is within the same prayer. This is what some scholars have favored in order to revive all the Sunnahs.

As for combining different forms of the tashahhud within a single recitation, there is a difference of opinion. Some scholars are of the view that it is preferable to use one form at one time and another form at another time, without combining them. Others believe that combining them is an innovation, as it was not transmitted from the Salaf.

Those who oppose combining the forms argue that it is a newly introduced method not preceded by any of the Imams, and that anyone who does it would logically have to combine all the reported types in all adhkar, which is invalid. Furthermore, the Prophet, peace and blessings be upon him, did not combine these different wordings at one time, and the intended meaning is achieved when expressed by any of the phrases. One wording is a substitute for the other, so it is not recommended to combine a substitute with what it substitutes.