Can it be said that the preponderant opinion is the establishment of the Tashahhud in the prostration of forgetfulness (sujūd al-sahw), based on the statement of Ibn Masʿūd (may Allah be pleased with him), even though the authentic hadiths do not mention it?

The issue of reciting the Tashahhud after the two prostrations of forgetfulness (Sujud al-Sahw) is a point of disagreement among jurists. The preponderant opinion is that there is no harm for one who recites the Tashahhud after the two prostrations of forgetfulness, nor is there any harm for one who omits the Tashahhud and makes the Taslim (salutation of peace).

The Ahadith narrated by Ibn Mas'ud concerning the prostrations of forgetfulness do not mention the Tashahhud after the prostrations. If the Tashahhud were prescribed, the Prophet, peace and blessings be upon him, would have performed it and commanded it.

The statement attributed to Ibn Mas'ud regarding the Tashahhud after the two prostrations of forgetfulness (mawquf) likely represents his own ijtihad (independent reasoning) and is not a narration directly from the Prophet, peace and blessings be upon him.

In conclusion: the matter in this issue is broad.