All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Al-Mawsoo‘ah Al-Fiqhiyyah Al-Kuwaytiyyah (Kuwaiti Encyclopedia of Fiqh) reads:
"The preferred opinion according to the Hanafis and Hanbalis is that Sujood as-Sahw is obligatory; the Hanbalis maintained that Sujood as-Sahw is obligatory for omitting any of the obligations of the prayer whose omission renders the prayer invalid…. The Maalikis, on the other hand, maintained that Sujood as-Sahw is a Sunnah whether performed before or after the Tasleem. This is the known opinion in the Maaliki school…. This scholarly opinion is also adopted by the Shaafi‘is and it is also one of the reported opinions (from Imaam Ahmad) in the Hanbali school …." [Summarized]
Allaah Knows best.