Full Idea
The Cogito is a transcendental argument; Descartes doesn't claim that it is a priori that he exists, but that any doubt or denial that he exists would presuppose his existence.
Gist of Idea
The Cogito is a transcendental argument, not a piece of a priori knowledge
Source
comment on René Descartes (Meditations [1641], §2.26) by Georges Rey - Contemporary Philosophy of Mind 3.2.1
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.78