Reward for reciting: ‘Raditu billahi Rabba…’
Al-Mundhir رضي الله عنه, the Companion of Nabi ﷺ, said when he was in North Africa, “I heard the Messenger of Allah ﷺ say, ‘If anyone says in the morning “I am pleased with Allah as a Lord, Islam as a deen and Muhammad ﷺ a Prophet” I am the leader who will take his hand to admit him to the Garden.”
This Hadith is recorded by Imam Tabarani (rahimahullah) in Al-Mu’jamul Kabir, Hadith:  838 with a weak chain.
However, it is sufficiently supported by other similar authentic Hadiths.
(Sahih Muslim, Hadith: 1884 and Sunan Abi Dawud, Hadith: 1524. Also see Al-Futuhatur Rabbaniyyah, vol.1 pg.256)
In fact Hafiz Ibn Hajar (rahimahullah) has cited another supportive chain for this very Hadith.
This seems to be the reason for which ‘Allamahs Mundhiri and Haythami (rahimahumallah) actually declared the Hadith in question as sound (hasan).
(Targhib, vol.1 pg.453, Majma’uz Zawaid, vol.10 pg.116 and Al-Isabah, number: 8252)
Transliteration of the mentioned du’a
Raditu billahi Rabba wabil Islami dina wa bi Muhammadin Rasula
Note: The name of this Sahabi (radiyallahu’anhu) has been spelt as: Al-Munaydhir and/or Al-Munyadhir by Hafiz Ibn Hajar in his book on the biographies of the Sahabah (radiyallahu’anhum).
And Allah Ta’ala Knows best.