The work of Imaam a-Laalikaa'ee (d. 418H) is one of the monumental compilations that document the creed of the Salaf us-Saalih in the first three centuries of Islaam, and the author brings over 2800 narrations from the Salaf with his chains of narration. Please refer to (this article) for more details on the book.

The book is firmly established as the work of al-Laalikaa'ee, with plenty of manuscripts present, and it has been mentioned as being the work of al-Laalikaa'ee by many scholars including Ibn Hajar al-Asaqlaanee, adh-Dhahabee, Ibn Taymiyyah and as-Suytootee. This is one of the greatest works compiled in aqidah, it is a tremendous reference for anyone wanting to know the creed of the Salaf.

What is Most Obligatory Upon a Muslim?

Imaam al-Laalikaa'ee (rahimahullah) said after his opening introduction and khutbah to the book, and after testifying that none has the right to be worshipped except Allaah and that Muhammad (alayhis salaam) is His Messenger:

Which translates:

To proceed. Then verily, that which is most obligatory upon a Muslim is knowledge of [the aspects of] the creed of the deen [of Islaam] and what Allaah has obligated upon His Servants including the understanding of His Tawheed and of His Attributes and believing in His Messengers with evidences and with certainty. And arriving at [all of] that and seeking evidences for them with clear proofs. And among the mightiest of statements and clearest of proofs and understandings is:

[1] The Book of Allaah, the Manifest Truth, [2], Then the saying of the Messenger of Allaah (sulAllahu 'alayhi wassalaam), [3] And of his Companions, the chosen, pious ones, [4] Then that which the Salaf us-Saalih were unanimously agreed upon, [5] The holding fast to all of that and remaining firm upon it till the Day of Judgement, [6] Then turning away from the innovations and from listening to them - from amongst those things the astray people have invented.

The foundation of Islam lies upon the beliefs held by the hearts, which are represented in the six pillars of faith (Eemaan) and these form the base and foundation. Having the correct belief is vital because the beliefs held in the heart give rise to the emotions of the heart (such as love, fear, hope), and in turn these emotions of the heart are the propelling factors behind a person's actions. Thus, to ensure that one's speech and action is valid, correct and acceptable to Allaah, the base and foundation, must be sound and correct. Deficiencies, and impurities in the base and foundation will manifest themselves in what is built upon it, which are the actions of the heart and one's outward speech and action. All of this affects nearness or distance of a person to his Lord and the quality of his worship. Therefore, this matter, - "knowledge of [the aspects of] the creed of the deen [of Islaam] and what Allaah has obligated upon His Servants including the understanding of His Tawheed and of His Attributes" - should be the utmost concern of a Muslim.