[وَاعْبُدْ رَبَّكَ حَتَّى يَأْتِيَكَ الْيَقِينُ ]
(And worship your Lord until the certainty comes to you) (15: 99).
Al-Bukhari said: "Salim said, '(This means) death.'''
This Salim is Salim bin `Abdullah bin `Umar.
Ibn Jarir also recorded from Salim bin `Abdullah,
[وَاعْبُدْ رَبَّكَ حَتَّى يَأْتِيَكَ الْيَقِينُ ]
(And worship your Lord until the Yaqin comes to you.) He said, "Death.''
It is reported in the Sahih from Umm Al-`Ala'- one of the women of the Ansar - that when the Messenger of Allah entered upon `Uthman bin Maz`un after he had died, Umm Al-`Ala' said, "May the mercy of Allah be upon you, Abu As-Sa`ib. My testimony over you is that Allah has honored you.'' The Messenger of Allah said,
«وَمَا يُدْرِيكَ أَنَّ اللهَ أَكْرَمَهُ؟»
(How do you know that Allah has honored him) I said, "May my father and mother be sacrificed for you, O Messenger of Allah! If not him, then who else'' He said,
«أَمَّا هُوَ فَقَدْ جَاءَهُ الْيَقِينُ، وَإِنِّي لَأَرْجُو لَهُ الْخَيْر»
(As far as he is concerned, the death has come to him, and I hope for good for him.) This is evidence that the meaning of this Ayah,
[وَاعْبُدْ رَبَّكَ حَتَّى يَأْتِيَكَ الْيَقِينُ ]
(And worship your Lord until the certainty comes to you.) is that acts of worship, such as prayer and the like, are obligatory on man so long as his mind is sound, so he should pray according to his best ability.
...
The Prophets - peace be upon them - and their companions, were the most knowledgeable of people about Allah, about His rights, His attributes, and the glorification that He deserves. But at the same time, they were the people who worshipped Him the most, continuing in good deeds until the time they died. Therefore,
what is meant by Yaqin here is death, as we have stated above. To Allah be praise and thanks. Praise be to Allah for His guidance. It is to Him that we turn for help and it is in Him that we put our trust. He is the One Whom we ask to help us to reach the best of circumstances, for He is the Most Generous and Kind. This is the end of the Tafsir of Surat Al-Hijr. Praise be to Allah, the Lord of all that exists.