What is the ruling on one who swore in writing on his mobile phone not to commit a specific sin, and then committed it?

An oath in writing is not binding unless there is an intention. If you did not intend it as an oath when writing it, then no expiation is required for committing the sin. However, if you intended to swear by Allah the Almighty, then an expiation for the oath is required. The expiation for an oath is feeding ten poor persons, or clothing them, or freeing a slave. If one cannot find any of these, then fasting for three days, as Allah the Almighty says: "So its expiation is the feeding of ten poor persons from the average of that which you feed your [own] families or clothing them or freeing a slave. But he who finds not [the means to do so] - then a fast of three days [is required]" [Al-Ma'idah: 89]. If your oath was a divorce, then it takes effect if you intended it at the time of writing. We advise you to hasten to sincere repentance and not return to what you have fallen into.