What is the ruling on voluntarily fasting on behalf of a living, healthy person who is unaware of it, with the intention of conveying the reward to them out of love?

It is not permissible for one to fast on behalf of another during their lifetime, by scholarly consensus. Scholars have differed regarding the permissibility of dedicating the reward of prayer and fasting to others and whether it reaches them. The majority of scholars hold that it is not permissible to dedicate their reward and that it does not reach the intended recipient. However, the Hanafis and Hanbalis are of the view that it is permissible and that the recipient benefits from it. Nevertheless, it was not the custom of the early generations (Salaf) to dedicate the reward of their deeds to others.