Is expiation incumbent upon one who swore, saying: "By God, if I stop praying, then my prayer is not for God or is for a specific person" – intending thereby to compel himself not to stop praying – and is expiation due immediately or upon returning to one's country if one is in a place devoid of settlers?

To proceed:

We warn you against abandoning prayer, as it is one of the gravest sins. What will help you maintain it are: striving against your soul, contemplating its virtue and the punishment for abandoning it, keeping company with the righteous, and making abundant supplications.

As for the oath you took, nothing whatsoever results from it, and it does not necessitate expiation if your intention was that abandoning prayer signifies that your previous prayers were not for the sake of Allah. Your previous prayers are valid and accepted if you performed them correctly and intended them as a means of drawing closer to Allah. Your subsequent abandonment of prayer does not affect their validity. However, you must repent and make up for the prayers you intentionally missed, according to the majority of scholars.