Is it permissible to use a specific, prescribed supplication for a particular occasion, such as the prayer before the Tasleem, on another occasion?

If the aforementioned supplication is made outside of prayer, then there is no objection to it; rather, it is desirable. However, if it is made inside the prayer, it has two cases: The first is if it is done intentionally, in which case the prayer is not invalidated, and no prostration of forgetfulness (sujood as-sahw) is required. The second is if it is done out of forgetfulness, in which case the prayer is likewise not invalidated. There is a difference of opinion regarding the obligation of the prostration of forgetfulness, and according to the view that it is legislated, it is on the basis of recommendation. As for the one praying behind the Imam, no prostration is required of him.