What should one do if they doubt whether they invoked blessings upon the Prophet in the Tashahhud during the prostration of forgetfulness (sujud al-sahw)?

Reciting the Salat al-Nabi (sending blessings upon the Prophet, peace and blessings be upon him) in the final tashahhud is a pillar [of the prayer]. If a worshipper doubts whether he has recited it, it is as if he has not recited it.

If the prostration of forgetfulness (sujud al-sahw) is performed before the taslim (salutation of peace), the worshipper should return to recite the Salat al-Nabi, then perform the prostration of forgetfulness, and then conclude with the taslim.

However, if the prostration of forgetfulness is performed after the taslim, the last rak'ah (unit of prayer) becomes void, and it is obligatory to perform a replacement rak'ah for it, then perform the prostration of forgetfulness. Otherwise, the entire prayer becomes void and must be repeated.

The length of time that renders rectification impossible is determined by common custom.