Should he fast on the fifteenth of Sha’baan even if the hadeeth is da'eef?

Is it permissible, even after finding out that a hadeeth is da'eef
(weak), to follow it, by way of doing righteous deeds? The hadeeth
says: "When it is halfwaythrough Sha'baan, spend that night in prayer
and fast on that day." The fast is observed as a voluntary act of
devotion to Allaah,as is spending that nightin prayer (qiyaam
al-layl).
Praise be to Allaah.
Firstly:
What is narrated concerning the virtue of praying, fasting and
worshipping on the fifteenth of Sha'baan (al-nusf min Sha'baan) does
not come under theheading of da'eef (weak), rather it comes under the
heading of mawdoo' (fabricated) and baatil (false). So it is not
permissible to followit or to act upon it, whether that is in doing
righteous deeds or otherwise.
A number of scholars ruled that the reports concerning that were
false, such as Ibn al-Jawzi in his book al-Mawdoo'aat, 2/440-445; Ibn
Qayyim al-Jawziyyah in al-Manaar al-Muneef, no.174- 177; Abu Shaamah
al-Shaafa'i in al-Baa'ith 'ala Inkaar al-Bida' wa'l-Hawaadith,
124-137; al-'Iraaqi in Takhreej Ihyaa' 'Uloom il-Deen, no. 582. Shaykh
al-Islam [Ibn Taymiyah] narrated that there was consensus on the fact
that they are false, in Majmoo' al-Fataawa, 28/138.
Shaykh Ibn Baaz (may Allaah have mercy on him) said in Hukm
al-Ihtifaal bi Laylat al-Nusf min Sha'baan (Ruling on celebrating the
fifteenth of Sha'baan):
Celebrating the night of the fifteenth of Sha'baan(Laylat al-Nusf min
Sha'baan) by praying etc, or singling out this day for fasting, is a
reprehensible bid'ah (innovation) according to the majority of
scholars, and there is no basis for this in sharee'ah.
And he (may Allaah have mercy on him) said:
There is no saheeh hadeeth concerning the night of the fifteenth of
Sha'baan (Laylat al-Nusf min Sha'baan). All the ahaadeeth that have
been narrated concerning that are mawdoo' (fabricated) and da'eef
(weak), and have no basis. There is nothing special about this night,
and no recitation of Qur'aan or prayer, whether alone or in
congregation, is specified for this night. What some of the scholars
have said about it being special is a weakopinion. It is not
permissible to single it out for any special actions. This is the
correct view. And Allaah is the Source of strength.
Fataawa Islamiyyah, 4/511.
See also question no. 8907 .
Secondly:
Even if we assume that the hadeeth is da'eef (weak) and not mawdoo'
(fabricated), the correct scholarly view is that weak ahaadeeth should
not befollowed at all, even if they speak of righteous deeds or of
targheeb and tarheeb (promises and warnings). The saheeh reports are
sufficient and the Muslimhas no need to follow the da'eef reports.
Nothing is known in Islam to suggest that thisnight or day is special,
either from the Prophet (peace and blessings of Allaah be upon him) or
from his companions.
The scholar Ahmad Shaakir said: There is no difference between rulings
or righteous deeds; we do not take any of them from da'eef reports,
rather no one hast he right to use any report as evidence unless it is
proven to be soundly narrated from the Messenger of Allaah (peace and
blessings of Allaah be upon him) in a saheeh or hasan hadeeth.
Al-Baa'ith al-Hatheeth, 1/278.
For more information, see: al-Qawl al-Muneef fi Hukm al-'Aml
bi'l-Hadeeth al-Da'eef.