Muwatta Malik 213
موطأ مالك#213SahihThese are the words of a Companion, not of the Prophet ﷺ. The chain is graded on its own terms.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عُمَرَ بْنِ مُحَمَّدِ بْنِ زَيْدٍ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَوَخَّ الَّذِي يَظُنُّ أَنَّهُ نَسِيَ مِنْ صَلاَتِهِ فَلْيُصَلِّهِ ثُمَّ لِيَسْجُدْ سَجْدَتَىِ السَّهْوِ وَهُوَ جَالِسٌ .
Yahya related to me from Malik from Umar ibn Muhammad ibn Zayd from Salim ibn Abdullah that Abdullah ibn Umar used to say, "If you are uncertain in the prayer, estimate what you think you have forgotten of the prayer and repeat it, then do the two sajdas of forgetfulness from the sitting position
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:
In-book reference: 3:46