al-Muwaṭṭaʾ (الموطأ) of Imām Mālik ibn Anas رحمه الله (d. 179 AH) is one of the most authoritive and also one of the earliest works in Ḥadīth literature focusing mainly on legal rulings. Within, you’ll find narrations of The Prophet ﷺ, his Companions, Tābʿīn, and some opinions of Imām Mālik رحمه الله himself.
Imām Mālik رحمه الله said regarding his Muwaṭṭaʾ:
عرضت كتابي هذا على سبعين فقيها من فقهاء المدينة، فكلهم واطأني عليه فسميته الموطأ
“I showed my book to seventy Fuqahā’ of Madīnah and all of them agreed with me on it, so I then named it al-Muwaṭṭaʾ.”1
Imām ash-Shāfiʿī رحمه الله (d. 204 AH) said about it before the advent of Ṣaḥīḥ al-Bukhārī and Ṣaḥīḥ Muslim:
لا أعلم كتاباً في العلم أكثر صواباً من كتاب مالك
“I do not know of any book that is more authentic than the book of Mālik.”2
It even reached a point to where the Caliph of his time, Abū Jaʿfar al-Manṣūr, wished to make al-Muwaṭṭaʾ the land’s constitutional law, to which Imām Mālik رحمه الله refused.3
Despite the work’s authority and imposing stature, it is not found amongst the six canonical books of Ḥadīth, which are Ṣaḥīḥ al-Bukhārī of Imām al-Bukhārī, Ṣaḥīḥ Muslim of Imām Muslim, Sunan Abū Dāwūd of Imām Abū Dāwūd, Sunan an-Nasā’ī of Imām an-Nasā’ī, Jāmiʿ at-Tirmidhī of Imām at-Tirmidhī, and Sunan ibn Mājah of Imām ibn Mājah. Shaykh Ṣāliḥ al-ʿUṣaymī حفظه الله explains the reasoning for this by saying:
“Why did the A’immah discard al-Muwaṭṭaʾ from the six main books of Ḥadīth despite it’s lofty status? Who will answer? (in question to his students) – It’s due to the reason that the Aḥādīth al-Aḥkām (Ḥadīth of Legal Rulings) in al-Muwaṭṭaʾ which are needed are already in the other six books. al-Bukhārī narrated from ʿAbdullāh ibn Yūsuf who narrates from Mālik, Muslim narrates from Yaḥyá ibn Yaḥyá who narrates from Mālik, Muslim also narrates from ʿAbdullāh ibn Maslamah who narrates from Mālik, and so on. Thus, the Aḥādīth which are needed regarding rulings in al-Muwaṭṭaʾ are already included in the six books, so it is not needed to be separately mentioned.”
May Allāh have mercy upon Imām Mālik & accept his efforts.
All praise is due to Allāh, may His peace & blessings be upon our final Prophet Muḥammad, his family, his companions, & all those who follow him in guidance.
 Tanwīr al-Hawālik P. 7 (Edit: this narration is Ḍaʾīf as within the Isnād is ʿAlī al-Khalanjī who is unknown)
 at-Tamhīd 1/76
 ad-Dībāj al-Madhhab 1/25
Ṣafīullāh Labīb ibn Salīm ʿAbd al-Malik
4th of Jumādá ath-Thānī 1444