Verse 34of 40

Certainty will Occur at the Time of Death. Allah Informs of the Condition at the Time of Death and What Terrors it Contains.May Allah make us firm at that time with the Firm Statement. Allah says,كَلاَّ إِذَا بَلَغَتِ التَّرَاقِىَ(Nay, when it reaches to the collarbones.) If we make the word "Kalla" negative, then this Ayah means, `O son of Adam! You are not able to deny that which I informed you of at that time (death). This will become something witnessed by you with your own eyes.

Source: Tafsir Ibn Kathir — abridged excerpt