Is it considered an innovation (bid‘ah) to specify the supplication “Allahumma inni as’aluka ‘aysh al-su‘ada’ wa mawt al-shuhada’ wa al-hashra ma‘a al-atqiya’ wa murafaqat al-anbiya’ (O Allah, I ask You for the life of the blessed, the death of martyrs, resurrection with the pious, and the companionship of prophets)” in the last prostration of supererogatory prayers?

Lengthening the last prostration (sujud (definition)) in prayer is not from the guidance of the Prophet, peace and blessings be upon him, for his prayer actions were of similar length, and prolonging them is contrary to the Sunnah (definition). Supplication can be made in any prostration, but singling out the last prostration for this is not legislated. It is better for supplication to be made after the final tashahhud (definition) and before the salutation (taslim (definition)).