Must one adhere to a single formula of Tashahhud in prayer, or is it permissible to vary it between the Tashahhud with the Imam and the worshipper's own Tashahhud?

All forms of tashahhud narrated from the Prophet, peace and blessings be upon him, are permissible, and whichever one a servant recites will suffice him. The scholars have agreed on the permissibility of all established tashahhuds. There is no harm in reciting one form of tashahhud at one time and another form at another time, even within a single prayer, as long as the obligatory part of the tashahhud is performed. Some scholars prefer that for acts of worship narrated in different forms, it is sunnah for a Muslim to perform one form sometimes and another form at other times; this is for the revival of the Sunnah, for the Sunnah to remain vibrant, and for the heart to be present when performing the Sunnah.