Imam Abd ar-Rahman ibn Mahdi {May Allah have mercy upon him} said:
ما نعرف كتابا في الإسلام بعد كتاب الله أصح من موطأ مالك
“We do not know of a book in Islam, after the Book of Allah, more authentic than the Muwatta of {Imam} Malik.”
{Related in the Siyar A’lam an-Nubala’, The Lives of the Noble Figures, Volume 9 pg. 205}