Full Idea
The certainty that 'I myself am in that I will die' is the basic certainty of Dasein itself. It is a genuine statement of Dasein, while 'cogito sum' is only a semblance of such a statement.
Clarification
'Dasein' roughly means 'existence' or 'being there'
Gist of Idea
Certainty that I will die is more basic to my existence than the Cogito
Source
Martin Heidegger (History of the Concept of Time [1925], p.316-7), quoted by Richard Polt - Heidegger: an introduction 4.§46-53
Book Reference
Polt,Richard: 'Heidegger: an introduction' [Routledge 2003], p.87
A Reaction
This just seems to be false. Children face their existence in thought long before they face their mortality. Absorption in activity can marginalise death, but not marginalise thought.