Muhammad ibn al-Husayn al-Ajurri (280-360 H) said:
“An inquirer asked about the knowledge that is obligatory for a Muslim to know and act upon—knowledge of which ignorance is unacceptable, and for which one will not be excused if they remain ignorant of it. The inquirer wished to learn about this to gain the desire to seek the indispensable knowledge, fearing that they might otherwise pursue sciences while others are more deserving of priority. And Allah is the Guardian of success.
The answer—and with Allah lies the success to arrive at correctness in speech and knowledge:
Know—may Allah have mercy on us and you—that it is obligatory upon every Muslim: sane, mature, rich or poor, noble or common, free or enslaved, male or female, healthy or chronically ill, to possess:
• A) The knowledge of knowing Allah, the Immaculate, with His Attributes.
• B) The knowledge of what Allah has obligated them to do regarding His worship, fulfilling His mandatory duties, and avoiding His prohibitions.
• C) The knowledge of sincerity (Ikhlas) to Allah in all aspects of worship, so that it is done purely for Allah and not for anyone else.
• D) The knowledge of knowing their enemy, the Devil, so that they may take him as an enemy.
• E) The knowledge of knowing their own souls, which prone to commanding evil, despite good intentions.”
— Fard Talab al-Ilm [1]
Knowledge where ignorance can be an excuse:
Muhammad ibn al-Husayn al-Ajurri said:
“If someone asks: ‘What is the knowledge of which a person is excused for being ignorant?’
It is said to him: This phrasing contains a degree of harshness/misplaced focus. However, your answer should be framed as follows:
Engaging in the pursuit of the knowledge we previously mentioned is an obligation upon you. If seeking this obligatory knowledge feels burdensome to you, while it feels easy for you to seek other types of knowledge—such as the history of the Children of Israel, stories of the Prophets, accounts of the Caliphs and the disputes that occurred between them, and similar matters—then it is said to you:
‘O heedless one! If you were ignorant of these latter things, your ignorance of them would not harm you. But if you are ignorant of what is mandatory for you to know and act upon, you will not be excused for your ignorance, and you would be disobedient to Allah by ignoring what is obligatory upon you.'”
— Fard Talab al-Ilm [24]