Sunan Ibn Majah 1218
سنن ابن ماجه#1218Sahihحَدَّثَنَا أَبُو بَكْرِ بْنُ خَلاَّدٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، أَنَّ ابْنَ مَسْعُودٍ، سَجَدَ سَجْدَتَىِ السَّهْوِ بَعْدَ السَّلاَمِ وَذَكَرَ أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ فَعَلَ ذَلِكَ .
It was narrated from ‘Alqamah that Ibn Mas’ud prostrated twice for the prostrations of forgetfulness after the Salam, and he mentioned that the Prophet (ﷺ) did that.”
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih
In-book reference: 5:416