He wants to continue fasting so that he can give up smoking

If what ismeant is fasting every day, so that you continue fasting continuously withoutbreaking the fast except on days when fasting is forbidden, such as the twoEids and the days of at-tashreeq (the three days following Eid al-Adha), thenthe more correct scholarly view is that it is makrooh and is not allowed.

But if whatyou meant is continuing to fast for part of the year, there is nothing wrongwith that, as it is confirmed by the Sunnah. Al-Bukhaari (1806) and Muslim(1890) narrated from ‘Aa’ishah (may Allah be pleased with her) that Hamzah ibn‘Amr al-Aslami asked the Messenger of Allah (blessings and peace of Allah beupon him): O Messenger of Allah, I am a man who fasts continually for periodsof time; can I fast when travelling? He said: “Fast if you wish and do not fastif you wish.”

An-Nasaa’i(2319) narrated from Usaamah ibn Zayd that the Messenger of Allah (blessingsand peace of Allah be upon him) used to fast continuously until it was saidthat he would not break the fast, and then he refrain from fasting until it wassaid that he would not fast.

Classed assaheeh by al-Albaani in Saheeh an-Nasaa’i.

Al-Bukhaari(1833) and Muslim (1956) narrated that ‘Aa’ishah (may Allah be pleased withher) said: The Messenger of Allah (blessings and peace of Allah be upon him)used to fast until we said that he would not break the fast, and he used torefrain from fasting until we said that he would not fast. And I never saw theMessenger of Allah (blessings and peace of Allah be upon him) fast any month infull except Ramadan, and I never saw him fast more than he did in Sha‘baan.

Shaykhal-Islam Ibn Taymiyah (may Allah have mercy on him) said:

With regard tofasting continuously for part of the year, this is something that the Prophet(blessings and peace of Allah be upon him) used to do. He would fast until onewould think that he would not break the fast, and he would refrain from fastinguntil one would think that he would not fast. Similarly, he would spend someentire nights in prayer (qiyaam), such as the last ten nights of Ramadan, andhe would spend other nights in prayer occasionally. This is what is mentionedin as-Sunan.

End quote fromMajmoo‘ al-Fataawa, 22/304

Better thanthat is the fast of the Prophet of Allah Dawood (peace be upon him). He used tofast one day and not the next. Al-Bukhaari (1841) and Muslim (1962) narratedfrom ‘Abdullah ibn ‘Amr (may Allah be pleased with him): News reached theProphet (blessings and peace of Allah be upon him) that I was fastingcontinually and spending every night in prayer. Either he sent for me or I methim and he said: “Have I not been informed that you fast and do not break thefast, and that you pray? Fast (some days) and do not fast (some other days),pray (qiyaam) for part of the night and sleep. For your eyes have a right over youand your soul has a right over you and your family has a right over you.” Hesaid: I have the strength to do that. He said: “Then observe the fast ofDawood, peace be upon him. He said: How (did he fast)? He said: “He used tofast one day and not fast one day, and he did not flee if he met (the enemy inbattle).”

According toanother report, “It is the proper way of fasting.” Narrated by al-Bukhaari(3165) and Muslim (1962)

According toanother report, “There is nothing better than that.” Narrated by al-Bukhaari(1840) and Muslim (1962)

Ibn al-Qayyim(may Allah have mercy on him) said:

It is provenin as-Saheehayn that the Prophet (blessings and peace of Allah be uponhim) said: “The best kind of fast is the fast of Dawood.” According to anotherversion: “There is nothing better than the fast of Dawood; he used to fast oneday and not the next.” This clear saheeh text that dispels all doubt shows thatfasting one day and not the next (i.e., fasting alternate days) is better thanfasting continually.

End quote fromTahdheeb as-Sunan, 7/71

But the bestadvice for each individual is that he should choose for himself the naafil actsof worship that he is able to do and persist in, and he should not take uponhimself that which will prevent him from doing other kinds of good deeds.

So look atwhat you are able to do and strive to do it; seek to do naafil acts of worshipthat are proven in reports from the Prophet (blessings and peace of Allah beupon him), with regard to fasting and other deeds.

With regard togiving up smoking, that is a good thing and we encourage you to go ahead withit; be determined and be firm with yourself concerning it. There is no doubtthat fasting will help you to a great extent with that. Try to keep yourselfbusy with acts of worship and find a good environment so that you will not betempted to go back to this bad habit.

See also theanswer to question no. 47565

And Allah knows best.