Tafsir of Al-Mu'minoon 23:111

(Indeed, I have rewarded them this day for what they endured patiently.) That is, because of their patience regarding your harm. This is an initiation [of a statement] to clarify their excellent state and that they benefited from what you inflicted upon them. It also contains a provocation towards the [disbelievers].

Regarding His, the Glorified’s, saying: (Indeed, it is they who are the successful):

It is either in the position of a second object for [the verb] "rewarded," as it takes a double object—both directly and via the letter ba—as Al-Raghib stated. That is: "I rewarded them with their success in obtaining the totality of their desires," as indicated by the object of the participle, while they are exclusively distinguished by that, as indicated by the insertion of the pronoun of separation (damir al-fasl).

Or, it is in the genitive case due to an implied causal lam (i.e., for the sake of their success through monotheism, which leads to every form of happiness). The fact that the reward is attributed to patience as a cause does not prevent this, because causes—since they are not complete causes—may be multiple.

Zayd ibn Ali, Hamzah, Al-Kisa’i, and Kharijah from Nafi’ read it as (innahum) with a kasra [on the hamza], based on the sentence being an initiation providing the reason for the reward. It has also been said: it is an explanation of the manner of the reward; so contemplate this.