What should he do if he heard the adhaan for Fajr and was having intercourse with his wife and did not withdraw?

Firstly:

It is obligatory to stop eating, drinking, having intercourse and doing anything else that may break the fast once the true dawn breaks, until the sun sets, because Allah, may He be exalted, says (interpretation of the meaning):

“And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]”

[al-Baqarah 2:187].

So if someone becomes certain that the true dawn has broken, he must stop [eating], and if he has something in his mouth, he must spit it out.

If the true dawn breaks whilst he is having intercourse and he withdraws immediately, his fast is valid and he does not have to do anything.

It is not permissible for him to continue having intercourse after becoming aware that dawn has broken; if he continues, then he invalidates his fast – and there is no scholarly disagreement concerning that – and he must make up that day and also offer expiation.

If his wife willingly went along with him, then she is subject to the same obligations as him: she must make up that fast and also offer expiation. If she refuses and he forces her to do that, then her fast is valid, and she does not have to do anything.

See the answers to questions no. 124290 and 106532.

Secondly:

There are signs of the true dawn by which it is known; the mu’adhdhins must make efforts to find the correct time.

Most mu’adhdhins nowadays rely on clocks and timetables, not on actually seeing the dawn, and they are not able to see it in cities because cities are filled with lights.

The adhaan that is based on clocks and timetables is not regarded as being certain with regard to the break of dawn, because of the well-known difference of opinion concerning the accuracy of these timetables, and the well-known view of more than one scholar which says that the adhaan that is based on timetables happens before the proper time; the extent of this error varies from one timetable to another.

Based on that, if someone eats or has intercourse at that time, thinking that there is still part of the night left, his fast is valid, because he was not certain that dawn had broken, especially if that happened shortly after the adhaan.

Undoubtedly what the Muslim who is honest with himself should do is be cautious with regard to matters of worship, and not be like the one who grazes his flock around a sanctuary, and soon transgresses onto it, in accordance with the words of the Prophet (blessings and peace of Allah be upon him): “Leave that which makes you doubt for that which does not make you doubt” and “Whoever keeps away from dubious matters will have protected his faith and his honour.”

What this implies is that he should stop eating, drinking, having intercourse, and doing anything else that breaks the fast, as soon as he hears the adhaan, even if he thinks that there is an error in the timetable, especially in the case of obligatory fasts. Making sure that acts of worship are correct and valid, and being keen to uphold them, is an important matter; the difference of opinion concerning timetables is something that is a well-known thorny issue. No one needs to put himself in such a situation; rather the wise person will be cautious with regard to his fast, and will stop eating, drinking and so on when he hears the adhaan; and he will be cautious with regard to his prayer, and will delay it until he knows for sure that the true dawn has broken.

Please see the answer to question no. 66202.

Conclusion:

If the mu’adhdhins in your country rely on sighting the dawn for giving the adhaan – not on clocks and timetables, then it is obligatory to stop having intercourse when hearing the adhaan. If a person does not stop immediately, at that exact moment, then he has spoiled his fast, and he must make up the fast and offer expiation, even if he does not ejaculate.

If he withdraws but continues after that to engage in some kind of intimacy other than vaginal intercourse until he ejaculates, then he has spoiled his fast and must make up that day, because he spoiled his fast by ejaculating, but he does not have to offer expiation, because expiation is required in the case of intercourse, but he stopped having intercourse when he heard the adhaan.

Please see the answer to question no. 71213.

If the mu’adhdhins rely on clocks and timetables, and he continued to have intercourse after hearing the adhaan for a short while during which he was not certain that dawn had broken, then his fast is valid, in sha Allah, but it would have been better to err on the side of caution regarding his fast.

And Allah knows best.