Jami` at-Tirmidhi 391
There is a narrated with another chain that Abu Hurairah and  ['Abdullah bin] As-Sa'id Al-Qari' would perform the prostrations of As-Sahw before the Taslim.

Abu 'Eisa said:
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.
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا عَبْدُ الأَعْلَى، وَأَبُو دَاوُدَ قَالاَ حَدَّثَنَا هِشَامٌ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ، أَنَّ أَبَا هُرَيْرَةَ، وَالسَّائِبَ الْقَارِئَ، كَانَا يَسْجُدَانِ سَجْدَتَىِ السَّهْوِ قَبْلَ التَّسْلِيمِ ‏.‏ قَالَ أَبُو عِيسَى حَدِيثُ ابْنِ بُحَيْنَةَ حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏ وَالْعَمَلُ عَلَى هَذَا عِنْدَ بَعْضِ أَهْلِ الْعِلْمِ وَهُوَ قَوْلُ الشَّافِعِيِّ يَرَى سُجُودَ السَّهْوِ كُلِّهِ قَبْلَ السَّلاَمِ وَيَقُولُ هَذَا النَّاسِخُ لِغَيْرِهِ مِنَ الأَحَادِيثِ وَيَذْكُرُ أَنَّ آخِرَ فِعْلِ النَّبِيِّ صلى الله عليه وسلم كَانَ عَلَى هَذَا ‏.‏ وَقَالَ أَحْمَدُ وَإِسْحَاقُ إِذَا قَامَ الرَّجُلُ فِي الرَّكْعَتَيْنِ فَإِنَّهُ يَسْجُدُ سَجْدَتَىِ السَّهْوِ قَبْلَ السَّلاَمِ عَلَى حَدِيثِ ابْنِ بُحَيْنَةَ ‏.‏ وَعَبْدُ اللَّهِ ابْنُ بُحَيْنَةَ هُوَ عَبْدُ اللَّهِ بْنُ مَالِكٍ وَهُوَ ابْنُ بُحَيْنَةَ مَالِكٌ أَبُوهُ وَبُحَيْنَةُ أُمُّهُ ‏.‏ هَكَذَا أَخْبَرَنِي إِسْحَاقُ بْنُ مَنْصُورٍ عَنْ عَلِيِّ بْنِ عَبْدِ اللَّهِ بْنِ الْمَدِينِيِّ ‏.‏ قَالَ أَبُو عِيسَى وَاخْتَلَفَ أَهْلُ الْعِلْمِ فِي سَجْدَتَىِ السَّهْوِ مَتَى يَسْجُدُهُمَا الرَّجُلُ قَبْلَ السَّلاَمِ أَوْ بَعْدَهُ فَرَأَى بَعْضُهُمْ أَنْ يَسْجُدَهُمَا بَعْدَ السَّلاَمِ ‏.‏ وَهُوَ قَوْلُ سُفْيَانَ الثَّوْرِيِّ وَأَهْلِ الْكُوفَةِ ‏.‏ وَقَالَ بَعْضُهُمْ يَسْجُدُهُمَا قَبْلَ السَّلاَمِ ‏.‏ وَهُوَ قَوْلُ أَكْثَرِ الْفُقَهَاءِ مِنْ أَهْلِ الْمَدِينَةِ مِثْلِ يَحْيَى بْنِ سَعِيدٍ وَرَبِيعَةَ وَغَيْرِهِمَا وَبِهِ يَقُولُ الشَّافِعِيُّ ‏.‏ وَقَالَ بَعْضُهُمْ إِذَا كَانَتْ زِيَادَةً فِي الصَّلاَةِ فَبَعْدَ السَّلاَمِ وَإِذَا كَانَ نُقْصَانًا فَقَبْلَ السَّلاَمِ ‏.‏ وَهُوَ قَوْلُ مَالِكِ بْنِ أَنَسٍ ‏.‏ وَقَالَ أَحْمَدُ مَا رُوِيَ عَنِ النَّبِيِّ صلى الله عليه وسلم فِي سَجْدَتَىِ السَّهْوِ فَيُسْتَعْمَلُ كُلٌّ عَلَى جِهَتِهِ يَرَى إِذَا قَامَ فِي الرَّكْعَتَيْنِ عَلَى حَدِيثِ ابْنِ بُحَيْنَةَ فَإِنَّهُ يَسْجُدُهُمَا قَبْلَ السَّلاَمِ وَإِذَا صَلَّى الظُّهْرَ خَمْسًا فَإِنَّهُ يَسْجُدُهُمَا بَعْدَ السَّلاَمِ وَإِذَا سَلَّمَ فِي الرَّكْعَتَيْنِ مِنَ الظُّهْرِ وَالْعَصْرِ فَإِنَّهُ يَسْجُدُهُمَا بَعْدَ السَّلاَمِ وَكُلٌّ يُسْتَعْمَلُ عَلَى جِهَتِهِ ‏.‏ وَكُلُّ سَهْوٍ لَيْسَ فِيهِ عَنِ النَّبِيِّ صلى الله عليه وسلم ذِكْرٌ فَإِنَّ سَجْدَتَىِ السَّهْوِ قَبْلَ السَّلاَمِ ‏.‏ وَقَالَ إِسْحَاقُ نَحْوَ قَوْلِ أَحْمَدَ فِي هَذَا كُلِّهِ إِلاَّ أَنَّهُ قَالَ كُلُّ سَهْوٍ لَيْسَ فِيهِ عَنِ النَّبِيِّ صلى الله عليه وسلم ذِكْرٌ فَإِنْ كَانَتْ زِيَادَةً فِي الصَّلاَةِ يَسْجُدُهُمَا بَعْدَ السَّلاَمِ وَإِنْ كَانَ نُقْصَانًا يَسْجُدُهُمَا قَبْلَ السَّلاَمِ ‏.‏