What is the expiation for vowing to pray for someone daily for their success and happiness before sleeping, and is expiation necessary if sleep overwhelms one and prevents them from supplicating?

This vow falls under the category of permissible vows, and according to the majority of scholars, nothing is incumbent upon the one who made it. However, according to the Hanbalis, the one who made the vow is given a choice between fulfilling the vow or offering an expiation for an oath (kaffarat yamin (definition)). The first opinion is the more preponderant one, but the more cautious approach is to adhere to the expiation for an oath, which is: freeing a slave, or feeding ten needy persons, or clothing them. And whoever cannot find these, then he should fast for three days.