What appears in al-Zarqā’s al-Madkhal al-Fiqhī (1/89) is the following:

“Abū Jaʿfar al-Manṣūr—and after him Hārūn al-Rashīd—both considered adopting the school of Imam Mālik and his book al-Muwaṭṭaʾ as the judicial code of the ʿAbbāsid state. Mālik forbade them to do so and said, ‘The Companions of the Messenger of Allah ﷺ disagreed over subsidiary matters and dispersed throughout the lands, and each of them is correct.’”

I say: this story is well known and widely circulated about Imam Mālik , but the closing words—“and each is correct”—have no basis in any of the reports or sources I have examined (1), save for a single narration recorded by Abū Nuʿaym in al-Ḥilya (6/332) whose chain contains al-Miqdām ibn Dāwūd, a transmitter whom al-Dhahabī lists among the weak (al-ḍuʿafāʾ). Even so, its wording is: “and each, in his own view, is correct.” The phrase “in his own view” shows that the version cited in al-Madkhal is defective. How could it be otherwise when it contradicts what trustworthy narrators have transmitted from Imam Mālik—that the truth is one and does not branch into multiple, conflicting forms, as has already been demonstrated? The same conviction is held by all the imams among the Companions, the Successors, the four mujtahid imams, and others.

Ibn ʿAbd al-Barr (2/88) said: “Had correctness existed in two mutually opposing views, the early generations would not have declared one another mistaken in their ijtihād, their judgments, and their legal opinions. Sound reason refuses to accept that a thing and its opposite can both be right.”