What is the ruling on the 'Asr prayer if the Imam forgot the last Tashahhud, then stood up, then returned to recite the Tashahhud and performed the prostrations of forgetfulness (sujood as-sahw) before the tasleem?

The prayer is valid. The jurists differed regarding someone who prays five rak'ahs, whether he should perform the prostration of forgetfulness (sujud as-sahw) before or after the salutation (taslim), with two opinions, both being narrations from Imam Ahmad. Regardless of whether he prostrates before or after the salutation, the prayer is valid.