the Messenger ﷺ mentioned: “Whoever fasts Ramadan, then follows it with six days of Shawwal, it is as though he had fasted a lifetime.” (1) This is because the wording of the hadith is, “Whoever fasts Ramadan,” whereas one who still owes make-up fasts cannot be said to have fasted Ramadan. This is clear.
Some students of knowledge have assumed that the disagreement over the validity of voluntary fasting before making up missed fasts applies to this case, but it does not. Rather, it does not apply here, because the hadith is explicit: the six days may only be fasted after making up the missed fasts of Ramadan.
The evidence that it is permissible to delay making up the fasts is the saying of Allah Most High: “But whoever is ill or on a journey, then let them fast an equal number of days later.” [al-Baqarah: 185]
As for the evidence that they must not be delayed until after the following Ramadan, it is as follows:
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The hadith of Aishah, may Allah be pleased with her, who said: “I would have fasts to make up from Ramadan, but I would not be able to make them up except in Shaban.” (2) Her statement, “I would not be able to make them up except in Shaban,” indicates that they must not be delayed until after Ramadan. The ability meant here is legal ability; that is, “I was not legally able to do so.”
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If one delays them until after Ramadan, he becomes like one who delays an obligatory prayer until the time of the next prayer without an excuse. It is not permissible to delay an obligatory prayer