Or if he forgets it, then on the next day at its time after making it up upon waking or upon remembering it, this is a matter of virtue, not a matter of strict obligation and requirement, since the Prophet, peace and blessings be upon him, has informed that the expiation for forgetting the prayer or sleeping through it is that the one who slept should pray it when he remembers it, and he has informed that there is no expiation for it except that.

Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) was asked about a person who sleeps through the prayer or forgets it (so what should he do?). The Prophet (peace and blessings be upon him) said: "Its expiation is that when he remembers it, he should perform it."