All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The Muwatta' of Imaam Maalik is one of the greatest books of Hadeeth and it is older than the Saheeh Al-Bukhari and the Saheeh Muslim. During his time, there were no books of Hadeeth greater than it before Saheeh Al-Bukhari and Saheeh Muslim were written, to the extent that Imaam Ash-Shaafi‘i who was a student of Imaam Maalik said about it: "There is no book after the Book of Allah that is more authentic than the book of Maalik."; another narration reads, "that is better than the book of Maalik."
As regards whether the Muwatta' includes Dha‘eef (weak) and Saheeh (authentic) Ahaadeeth, then you should know that the contents of the Muwatta' are divided into three categories which were classified by the scholars as follows:
1. Muttasil Ahaadeeth: which are Ahaadeeth that have an uninterrupted chain of narrators to the Prophet . All of such Ahaadeeth in the Muwatta' are Saheeh. In fact, the scholars said that they are like the Ahaadeeth reported by Al-Bukhari and Muslim in terms of authenticity, as stated by As-Sayooti in his commentary on the Muwatta'.
2- Mursal Ahaadeeth: It is when a Taabi‘i (Successor of the Companions) says, "The Prophet said…." Imaam Maalik considered Mursal Ahaadeeth to be authoritative and some scholars are of the view that the Mursal Ahaadeeth of Imaam Maalik are more authoritative than the Mursal Ahaadeeth reported by the Taabi‘is although he (Maalik) was from the generation of the successors of the Taabi‘is. That is because he was very stringent in regard to the chain of narrators, and he was the most knowledgeable person of the narrators of Hadeeth in Madeenah.
3- Balaagh Ahaadeeth (or Balaaghaat): Sometimes Imaam Maalik would say in the Muwatta', "It reached me that the Messenger of Allah said…." He may also say: "It reached me from so and so (a Companion) that…." This means that he dropped two or more men from the chain of narrators, and this, according to the scholars, means that the Hadeeth is not Muttasil (connected in its chain of narration to the Prophet ). For example, Imaam Maalik says: "It reached me that the Prophet prohibited wearing gold rings."
The scholars said that the Mursal Ahaadeeth and Balaaghaat in Maalik's Muwatta' all have complete chains of narrators in narrations reported by other than Maalik.
Al-Haafith ibn ‘Abdul Barr reported them with other chains of narrators connected to the Prophet in his book At-Tamheed lima fi Al-Muwatta' min Al-Ma‘aani wal-Asaaneed; with the exception of four Ahaadeeth whose chains of narrators he could not find. Later, Al-Haafith ibn As-Salah reported them with their complete chain of narrators to the Prophet in a treatise that he called Wasl Al-Balaaghaat Al-Arba‘ah fi Al-Muwatta'.
What distinguishes the Muwatta' of Maalik from other books of Hadeeth is the high (short) chain of narrators because Imaam Maalik was close to the era of the Prophet as he was from the generation that succeeded the Taabi‘is. His chain of narrators are higher than the chain of narrators of the authors of the six books of Hadeeth (i.e. Al-Bukhari, Muslim, Abu Daawood, At-Tirmithi, An-Nasaa'i, and Ibn Maajah) as he preceded them by two generations. The highest chain of narrators in the book of Maalik are the two-part chain of narrators, like the narrations from Naafi‘ from Ibn ‘Umar .
Allah Knows best.