All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is better to perform Sujood As-Sahw for this brief sitting, following the opinion that Sujood As-Sahw is required in this case. If you do not perform Sujood As-Sahw, we believe that your prayer is still valid and you are not liable for anything because it was a brief sitting.
Ibn Qudaamah said:
"If he sits in other than the place for the Tashahhud for a period of time equal to that of Jilsat Al-Istiraahah, then Al-Qaadhi said, 'He is required to perform Sujood As-Sahw regardless of whether we say that Jilsat Al-Istiraahah is an act of the Sunnah or not, because he did not intend to perform it when he sat; rather, he intended other than it, so it is an act of forgetfulness on his part (that requires Sujood As-Sahw).' It is also possible that he is not required to perform Sujood As-Sahw because it is an act which if he did intentionally, and it does not invalidate his prayer, so if he did it forgetfully, it does not require Sujood As-Sahw. This is similar to the situation when the praying person makes a slight movement during the prayer that is not related to the acts of the prayer..." [Al-Mughni]
The Hanbali scholar Al-Mardaawi said:
"If he sits out of forgetfulness in the place of Jilsat Al-Istiraahah for the average time of that Jilsah, then he is required to perform Sujood As-Sahw. This is one of the two scholarly views in this regard and the one that is most likely correct of them... The second view is that Sujood As-Sahw is not required. This was a possibility stated in Al-Mughni. Al-Haawiyayn reads, 'I believe that this is the most likely correct view.' Az-Zarkashi said, 'If he sits for a brief period of time, he is not required to perform Sujood As-Sahw.' At-Talkhees reads, 'This opinion is Qiyaas Al-Mathhab (the authoritative and standard analogy of our school of Fiqh), and what Al-Qaadhi mentioned (i.e. that he is required to perform Sujood As-Sahw in this case) is groundless...'"
Such a brief sitting in the odd units of prayer out of forgetfulness does not require Sujood As-Sahw according to the Maaliki scholars either. They held that Sujood As-Sahw is required only if the length of the sitting is equal to the sitting of the Tashahhud.
Allah knows best.