Muwatta Malik 188
موطأ مالك#188SahihThese are the words of a Successor — the generation after the Companions — not of the Prophet ﷺ.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّهُ كَانَ يَقْرَأُ خَلْفَ الإِمَامِ فِيمَا لاَ يَجْهَرُ فِيهِ الإِمَامُ بِالْقِرَاءَةِ .
Yahya related to me from Malik from Hisham ibn Urwa that his father used to recite behind the imam when the imam did not recite aloud
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-HilaliMaqtu Sahih
In-book reference: 3:21