Whoever misses an obligatory prayer because of sleep or forgetfulness has to make it up when he remembers it, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever forgets a prayer or sleeps and misses it, let him pray it when he remembers it, for there is no other expiation for it than that.”
Al-Bukhaari, 572; Muslim, 1564.
And it was proven that the Prophet (peace and blessings of Allaah be upon him) made up the Sunnah of Zuhr after praying ‘Asr. Narrated by al-Bukhaari, 4370; Muslim, 834.
Al-Nawawi said: This clearly refers to making up a missed Sunnah prayer, so it is more appropriate that missed obligatory prayers should be made up.
If a Muslim sleeps and misses a prayer, or he forgets it, and he does not remember until after ‘Asr, then he must pray it at that time.