Is the prayer valid if the imam performs only one prostration in the final tashahhud and says one salam, then prostrates a second time and says salam, without performing the prostrations of forgetfulness (sujud al-sahw)?

The Maliki and Hanbali schools of thought, regarding this issue, hold that a complete rak'ah must be performed. If the interval is long, the prayer becomes invalid and must be repeated. Whereas the Hanafi and Shafi'i schools of thought maintain the validity of the prayer as long as the interval is short, in which case only the prostration (sajdah) is performed, and the prayer of the Imam and those with him remains valid. Imam An-Nawawi stated that whoever is certain of having omitted a rak'ah or a pillar of prayer before a long interval is obligated to build upon his prayer, complete the remainder, and perform the prostration of forgetfulness (sujood as-sahw). However, if it is after a long interval, he is obligated to restart the prayer.