as Allah Most High said to His Prophet:

“Worship your Lord until certainty comes to you.” [Surah al-Hijr 99]

Al-Hasan al-Basrī said: “Allah has set no term for His believing servant short of death.” Some among these people have mistakenly believed that this means: “Worship your Lord until you attain knowledge, then abandon worship.” This is ignorance and misguidance by the consensus of the Muslim community. Rather, “certainty” here has the same meaning as in His words:

“Until certainty came to us.” [Surah al-Muddaththir 47]

It is reported in the Sahīh that when ʿUthmān ibn Mazʿūn died, the Prophet, may Allah bless him and grant him peace, said: “As for ʿUthmān, certainty has come to him from his Lord. By Allah, although I am Allah’s Messenger, I do not know what will be done with me.”

As for certainty as a quality of the servant, he has possessed it since he began worshipping his Lord. Worship is not valid without it, although it has varying degrees.

Allah Most High said: “Alif Lām Mīm. This is the Book about which there is no doubt, a guidance for the God-fearing,” up to His words: “And of the Hereafter they are certain.” [Surah al-Baqarah 1–4]