All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
To say: “Jaza Allaahu ‘anna Muhammadan ma huwa ahluhu (May Allah reward Muhammad, sallallaahu ‘alayhi wa sallam, for (guiding) us with befitting rewards),” is a good supplication, but it is not one of the reported wordings for invoking exalting the mention of the Prophet, sallallaahu ‘alayhi wa sallam. If a person makes an oath to recite this supplication, it counts as a binding oath, and either he has to fulfill it, or else it would be a broken oath and he would be obliged to offer Kaffaarah (expiation) for breaking it. If the wording of the oath was, for example, “By Allah, if such-and-such happens, I will say ‘Jazza Allaahu ‘ana Muhammadan, sallallaahu ‘alayhi wa sallam, ma huwa ahluhu’ 200 times; rather 500 times,” the oath would be dissolved by offering one expiation for breaking it. The repeated breaking of it does not necessitate giving multiple expiations, meaning that you are required to offer the expiation only once. Az-Zarkashi said in Al-Manthoor: “Whenever a person breaks the oath, it is dissolved, and he does not have to offer an expiation every time he breaks it (i.e. the oath of that incident).” [End of quote]
The expiation for breaking an oath was previously explained in Fatwa 102811.
Allah Knows best.