What is al-Shafi'i's evidence that all prostrations of forgetfulness come before the salam?

The timing of the prostration of forgetfulness (sujud (definition) al-sahw) is a point of disagreement among scholars, but they all agree on its validity whether performed before or after the salutation (taslim (definition)), with the disagreement lying in which is preferable. Our preferred view—which is the madhhab (definition) of Malik and some Shafi'is, and the choice of Shaykh al-Islam Ibn Taymiyyah—is that if it is due to a deficiency, its place is before the salutation, and if it is due to an addition, its place is after the salutation.

As for the well-known position of the Shafi'i madhhab, it is that all prostrations of forgetfulness are performed before the salutation. They cite as evidence hadiths, including those of Ibn Buhaynah and Abu Sa'id, and they rationalize this by stating that it was legislated to rectify the prayer, and thus it should be performed before exiting the prayer. Imam al-Shafi'i reconciled the various hadiths on this matter. He viewed the hadiths of Abu Sa'id and Abd al-Rahman ibn Awf as explicitly clarifying the ruling on forgetfulness, and these indicate that the prostration is before the salutation. However, he interpreted the hadith (definition) of Dhul-Yadain—in which the prostration was after the salutation—as indicating that its delay was due to forgetfulness, not intentional.