Muwatta Malik 344
موطأ مالك#344SahihThese 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 ابْنَ عُمَرَ، أَقَامَ بِمَكَّةَ عَشْرَ لَيَالٍ يَقْصُرُ الصَّلاَةَ إِلاَّ أَنْ يُصَلِّيَهَا مَعَ الإِمَامِ فَيُصَلِّيهَا بِصَلاَتِهِ .
Yahya related to me from Malik from Nafi that Ibn Umar stayed in Makka for ten nights, shortening the prayer, except when he prayed it behind an imam, in which case he followed the imam's prayer
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:
Salim al-HilaliMauquf Sahih
In-book reference: 9:19