It is one of the earliest surviving hadith compilations, written by Imam Malik ibn Anas (d. 179 AH) and arranged by chapters of fiqh, containing marfu' hadith alongside statements of the Companions and Successors and the settled practice of the people of Madinah. Imam ash-Shafi'i famously said he knew of no book on the face of the earth, after the Book of Allah, more authentic than the book of Malik — a statement made before al-Bukhari and Muslim compiled their Sahihs, which is how the later scholars understood it. Its material is overwhelmingly sound, but it does include mursal reports and balaghat ("it reached me that..."), and Ibn 'Abd al-Barr in at-Tamhid traced connected chains for all but a few of them. It is not counted among the six books in the standard reckoning, though some scholars, such as Razin ibn Mu'awiyah and Ibn al-Athir in Jami' al-Usul, included it as the sixth in place of Sunan Ibn Majah.
Q&A · Hadith
What is the Muwatta of Imam Malik, and what is its status among the hadith books?
Evidence — the daleel
Malik ibn Anas, al-MuwattaIbn 'Abd al-Barr, at-Tamhidash-Shafi'i's statement, transmitted by Ibn 'Abd al-Barr and others
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.