"The Prophet (S) stood for the Zuhr prayer, and he had a sitting to perform, so when he completed his Salat, he performed two prostrations, saying the Takbir for each prostration. So he was sitting before saying the Salam, and the people prostrated with him in place of the sitting he forgot."  

أَنَّ النَّبِيَّ ﷺ قَامَ فِي صَلاَةِ الظُّهْرِ وَعَلَيْهِ جُلُوسٌ فَلَمَّا أَتَمَّ صَلاَتَهُ سَجَدَ سَجْدَتَيْنِ يُكَبِّرُ فِي كُلِّ سَجْدَةٍ وَهُوَ جَالِسٌ قَبْلَ أَنْ يُسَلِّمَ وَسَجَدَهُمَا النَّاسُ مَعَهُ مَكَانَ مَا نَسِيَ مِنَ الْجُلُوسِ قَالَ وَفِي الْبَابِ عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ  

The Hadith of Ibn Buhainah is a Hasan [Sahih] Hadith, and this is acted upon according to some of the people of knowledge. It is the opinion of Ash-Shafi'i, he held the view that all prostrations for As-Sahw were to be performed before the Salam, saying: "This one abrogates the other Ahadith" and he mentioned that the last action of the Prophet ﷺ was according to this. Ahmad and Ishaq said: "When a man stands up after two Rak'ah, then he performs the prostrations for As-Sahw before the Salam according to the Hadith of Ibn Buhainah." 'Abdullah bin Buhainah is 'Abdullah bin Malik [so he is] Ibn Buhainah (because) Malik is his father and Buhainah is his mother. I was informed of this by Ishaq bin Mansur from 'Ali [bin 'Abdullah] bin Al-Madini. Abu 'Eisa said: The people of knowledge differ over when a man is to perform the prostrations of As-Sahw, is it before the Salam or after it. Some of them thought that her performs them after the Salam. This is the view of Sufyan Ath-Thawri and the people of Al-Kufah. Some of them said he performs them before the Salam. This is the view of most of the Fuqaha among the people of Al-Madinah, like Yahya bin Sa'eed, Rabi'ah, and others. This is also the saying of Ash-Shafi'i. Some of them said when he adds to the Salat, then it is after the Salam, and when he leaves something out, then before the Salam. This is the view of Malik bin Anas. Ahmad said: "Whatever is reported from the Prophet ﷺ about the prostrations from As-Sahw then it is acted upon in either case." He saw that when one stands after Rak'ah then according to the Hadith of Ibn Buhainah, he is to perform the prostrations before the Salam. When he prays five for Zuhr, then performs the prostrations after the Salam, and if he says Salam after two Rak'ahs of Zuhr or 'Asr then he performs the prostrations after the Salam. All of them are to be acted upon depending upon the case, and in the cases where nothing is reported from the Prophet ﷺ, then two prostrations are performed for As-Sahw before the Salam. Ishaq said the same as Ahmad about all of this, with the exception that he said that for every case of As-Sahw that is not mentioned from the Prophet ﷺ, then if it is an addition to the Salat, then prostrations are performed after the Salam, and if it is something that was left out, then the prostrations are performed before the Salam.  

أَنَّ أَبَا هُرَيْرَةَ وَالسَّائِبَ الْقَارِئَ كَانَا يَسْجُدَانِ سَجْدَتَىِ السَّهْوِ قَبْلَ التَّسْلِيمِ