Say three times -

رَضِيْتُ بِاللَّهِ رَبًّا وَبِالْإِسْلَامِ دِيْنًا وَبِمُحَمَّدٍ نَبِيًّا

Radheetu billaahi Rabban, wa bil-'Islaami deenan, wa bi-Muhammadin Nabiyyan

I am pleased with Allah as a Lord, and Islaam as a religion and Muhammad (ﷺ) as a Prophet.

Reference: Sahih li ghairihi (Shu’aib al-Arnaut). Takhrijul Musnad: 23112