Is the prostration of forgetfulness (sujud al-sahw) obligatory for someone who uttered only the word "peace" (al-salam) without completing the salutation or turning during the first tashahhud in the 'Isha prayer, performed alone, and if it is obligatory, should it be performed before or after the salutation?

Saying the word "As-salam" inadvertently during prayer necessitates the prostration of forgetfulness (sujud as-sahw), because it is considered speech. Imam An-Nawawi holds that speech, which intentionally invalidates the prayer, requires the prostration of forgetfulness if it occurs inadvertently. It is preferable for the prostration of forgetfulness to be performed after the salutation (taslim), according to many scholars, though it is permissible if performed before it.