According to the Hanbali madhab it is a obligatory action (at minimum to recite the supplication رب اغفر لي once) see page 1 and onwards of this PDF: http://www.ibnfarooq.com/Lessons/Lesson37[1].pdf but if it is left out forgetfully it can be compensaded by making sajdah sahw. I had always wondered why the Hanafis don't say anything between the sujud, since according to them one should sit still motionless for the duration of saying once "SubhanAllah", by reciting once or as in the hadith twice "Rabbighirli" this duration is fulfilled. But they dont recite it and most of them don't even sit still motionless in between the sujud, rather they come up from the sujud and go down again in one movement. And such performed salaah is not valid according to the Malikis, Shafis, Hanbalis and according to Abu Yusuf.