He apostatised during the day in Ramadan then he came back to Islam – what must he do?

Being Muslim is one of the essential conditions for the validity and acceptance of acts of worship. 

One of the indications of that is the verses in which Allah, may He be exalted, says (interpretation of the meaning):

“Say: ‘Spend (in Allah’s Cause) willingly or unwillingly, it will not be accepted from you. Verily, you are ever a people who are Fasiqoon (rebellious, disobedient to Allah).’

And nothing prevents their contributions from being accepted from them except that they disbelieved in Allah and in His Messenger (Muhammad (blessings and peace of Allah be upon him)); and that they came not to As-Salat (the prayer) except in a lazy state; and that they offer not contributions but unwillingly”

[at-Tawbah 9:53-54]. 

This verse clearly shows that the condition of an act of worship being accepted is faith, without which it is not valid. 

Ibn Katheer (may Allah have mercy on him) said:

The words “Say: ‘Spend (in Allah’s Cause) willingly or unwillingly” mean: No matter what you spend, willingly or unwillingly, “it will not be accepted from you. Verily, you are ever a people who are Fasiqoon (rebellious, disobedient to Allah).” 

Then Allah, may He be exalted, tells us of the reason for that, which is that it will never be accepted from them, is “that they disbelieved in Allah and in His Messenger (Muhammad (blessings and peace of Allah be upon him)” that is, they have disbelieved, and deeds are only valid when accompanied by faith.

End quote from Tafseer Ibn Katheer (4/162). 

Whoever disbelieves and apostatises during his fast has rendered his fast invalid. 

Ibn Qudaamah (may Allah have mercy on him) said: 

We do not know of any difference of opinion among the scholars concerning the fact that the one who apostatises from Islam whilst fasting invalidates his fast, and he must make up that day when he comes back to Islam, whether he becomes Muslim again during the day or after the day has ended, and whether his apostasy had to do with some belief that he held, that constituted disbelief (kufr), or doubting a belief where doubting that belief constitutes disbelief (kufr), or because he spoke words of disbelief, either by way of mockery or otherwise. Allah, may He be exalted, says (interpretation of the meaning):

“If you ask them (about this), they declare: ‘We were only talking idly and joking.’ Say: ‘Was it at Allah, and His Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) and His Messenger (blessings and peace of Allah be upon him) that you were mocking?’

Make no excuse; you have disbelieved after you had believed. If We pardon some of you, We will punish others amongst you because they were Mujrimoon (disbelievers, polytheists, sinners, criminals, etc.)”

[at-Tawbah 9:65-66]. 

End quote from al-Mughni (4/369-370) 

With regard to expiation, we have previously explained in fatwa no. 106476 that it is not required except in the case of one who spoils the Ramadan fast by having intercourse only. As for all other things that break the fast, nothing is required except making up that day only. 

Based on that, this man has to make up one day in place of the day that he spoiled, and he does not have to offer expiation. 

And Allah knows best.