He said, “May the curse of Allaah be upon me if I smoke another cigarette” then he smoked again

Firstly: It is not permissible for a man to swear against
himself and invoke the curse or wrath of Allaah upon himself, or swear an
oath by saying that he will be a kaafir or beyond the pale of Islam or he
will be an apostate and so on. The evidence for that is as follows: 

1 – The verse in which Allaah says (interpretation of the
meaning): 

“And were Allaah to hasten for mankind the evil (they
invoke for themselves and for their children, while in a state of anger) as
He hastens for them the good (they invoke) then they would have been ruined”

[Yoonus 10:11] 

al-Qurtubi said: 

Mujaahid said: This was revealed concerning a man who prayed
against himself, his wealth or his children when he got angry, saying, “O
Allaah, destroy it, O Allaah, do not bless it and curse it” etc. If that
were to be responded to as prayers for good were responded to, then they
would have been ruined. So this verse was revealed to criticize this bad
characteristic that exists in some people who pray for good and want a quick
response, then sometimes their bad attitude makes them pray for bad things,
but if those prayers were to be answered quickly they would be ruined. 

Tafseer al-Qurtubi, 8/315. 

2 – It was narrated from
Thaabit al-Dahhaak (may Allaah be pleased with him) that the Messenger of
Allaah (peace and blessings of Allaah be upon him) said: “Whoever
swears an oath by saying that he will belong to a religion other than Islam,
then he is as he said.”  Narrated by al-Bukhaari, 5700. 

3 – The Prophet (peace and blessings of Allaah be upon
him) heard a man cursing his camel when it walked too slowly. The Messenger
of Allaah (peace and blessings of Allaah be upon him) said: “Who is
cursing his camel?” He said: “It is me, O Messenger of Allaah.” He said:
“Get down from it, for we do not want to be accompanied by something that is
cursed. And do not pray against yourselves or against your children, and do
not pray against your wealth, lest you say such a thing and it coincides
with a time when Allaah answers prayers and responds to what you ask for.”
Narrated by Muslim, 3014. 

Secondly:  With
regard to what the one who has said such a thing should do, there is a
difference of opinion among the scholars. Some of them are of the view that
he has to offer expiation, and others are of the view that he does not have
to offer expiation. 

In al-Musannaf, ‘Abd al-Razzaaq narrated some reports
that were transmitted from the salaf concerning that: 

It was narrated from Ibn ‘Abbaas concerning a man who swore
that he would be a Jew or a Christian or a Magian or out of Islam or that
the curse of Allaah would be upon him or that he would be obliged to do
something, that this was a binding oath. (15974). 

It was narrated that Tawoos said: “Whoever says ‘I am a
kaafir’ or ‘I am a Jew or a Christian or a Magian’ or ‘May Allaah humiliate
me’ and the like (if I do such and such), this is an oath (yameen) for which
he must offer expiation.” (10975). 

It was narrated that Ibn Jurayj said: I heard a person say to
‘Ata’: “If a man says, May the wrath of Allaah be upon me, or May Allaah
humiliate me or I pray to Allaah against myself or …, will that put him
beyond the pale of Islam?” He said: “I would rather that he carried out what
he resolved to do. If he did not carry out what he resolved to do, then he
does not have to do anything because it is not an oath.” (15977). 

It was narrated that Ibn Jurayj said: I heard ‘Ata’ being
asked about a man who says: “I give my promise and covenant to Allaah,” then
he broke his word; was that an oath (yameen)? He said: “No, unless he
intended it as an oath or he said, May Allaah humiliate me, or may the curse
of Allaah be upon me, or he said I will associate someone else with Allaah
or I will disbelieve in Allaah and so on. He said: No (meaning, it is not an
oath), unless he swore by Allaah.” (15978). 

Ibn Qudaamah (may Allaah have mercy on him) said that it is
more likely that he does not have to offer expiation because there is no
evidence to suggest that offering expiation in this case is obligatory.  See
al-Mughni, 13/465. 

Shaykh ‘Abd al-Rahmaan ibn Jibreen (may Allaah preserve him)
was asked about this matter and he said: This is not an oath (yameen) and no
expiation is required, but he has to repent to Allaah from that. 

If the Muslim wants to be more on the safe side and offer
kafaarat yameen (expiation for breaking an oath), that is fine. 

And Allaah knows best.