Is the prostration of forgetfulness (sujud al-sahw) obligatory or recommended for someone who forgot to send blessings upon the Prophet (peace and blessings be upon him) in the first tashahhud and remembered it in the third rak'ah?

According to the Shafi'is, sending blessings upon the Prophet (peace and blessings be upon him) in the first tashahhud (definition) is a Sunnah (definition), and omitting it does not necessitate sujood as-sahw (prostration of forgetfulness). As for the majority of scholars, they do not consider it prescribed in the first tashahhud, and this is the stronger opinion. It is appropriate to limit oneself to reciting only the tashahhud in the middle sitting.