Is it obligatory for someone who caught three rak'ahs with the Imam and made taslim with him, then prayed the Sunnah (post-prayer voluntary prayer), upon being alerted by his neighbor, to pray one rak'ah, or to repeat the entire prayer because he separated it with the Sunnah?

If someone forgets something in their prayer and omits a pillar or a complete rak'ah, and then remembers after concluding the prayer with taslim:

If they remember before a long interval has passed, they must build upon their prayer, perform the remaining part, and then perform the prostration of forgetfulness (sujud al-sahw). If they remember after a long interval has passed, they must restart the prayer.

There is disagreement regarding the precise definition of a "long interval." Some say it refers to what is customary and usual, while others say it is the duration of a long or short rak'ah. The more cautious approach for someone who has separated by two rak'ahs is to restart the prayer.