Muwatta Malik 408
موطأ مالك#408SahihThese are the words of a Companion, not of the Prophet ﷺ. The chain is graded on its own terms.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ مَنْ نَسِيَ صَلاَةً فَلَمْ يَذْكُرْهَا إِلاَّ وَهُوَ مَعَ الإِمَامِ فَإِذَا سَلَّمَ الإِمَامُ فَلْيُصَلِّ الصَّلاَةَ الَّتِي نَسِيَ ثُمَّ لِيُصَلِّ بَعْدَهَا الأُخْرَى .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "Someone who only remembers that he has forgotten a prayer when he is praying the next prayer behind an imam, should pray the prayer he has forgotten after the imam has said the taslim, and then pray the other one again
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: 9:83