Shahada
الشَّهَادَةThe testimony of faith
The first and most fundamental pillar: to testify that there is no deity worthy of worship but Allah, and that Muhammad is His Messenger. It is the doorway into Islam and the ground of every act of worship. Accepted sincerely, it erases every sin that came before (Muslim 121) and makes a Muslim of whoever says it with conviction.
أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ
« I bear witness that there is no deity but Allah, and I bear witness that Muhammad is the Messenger of Allah. »
Bukhari 8, Muslim 16
Once in a lifetime at minimum, with conviction. In practice: recited in the tashahhud at every prayer, and daily in the adhkar.