Raḍītu billāhi Rabba,
wa bil-Islāmi dīna,
wa bi-Muḥammadin (ṣallallāhu `alayhi wa sallama) nabiyya.
I am pleased with Allah as my Lord,
with Islam as my religion,
and with Muhammad (peace and blessings of Allah be upon him) as my Prophet.
(Recite three times in Arabic.)
Reference:
"Allah has promised that anyone who says this three times every morning or
evening will be pleased on the Day of Resurrection." Ahmad 4/ 337,
An-Nasa'i, 'Amalul-Yawm wal-Laylah p. 4, Ibn As-Sunni (no. 68), At-Tirmidhi
5/465. Its chain of transmission is good (Hasan), Ibn Baz, p. 39.