The use of weak hadith is permissable and has been used by Sheikh Ibn Taymiaah as you will see from the evidence below;
Shaykh al-Islam Abu Zakariyya al-Nawawi (rad.ia-LLahu `anhu) said in the 'Adhkar': "The ulama among the experts in hadith and the experts in law and others have said: it is permissible and (also) recommended that the religious practice (al-`amal) concerning good deeds and good character (al-fadâ'il), encouragement to good and discouragement from evil (al-targhib wa al-tarhib) be based (even) on weak hadith (bi al-hadith al- da`îf) as long as it is not forged (mawdu`).
It has been reported from Imam Ahmad that one may practice on the basis of the weak hadith if there is no other hadith to that effect and also if there is no hadith that contradicts it. In one narration he is reported to say: "I like weak hadith better than men's opinions."
Ibn Taymiyya (Leader of Wahhabi) said in his book "al-qaida al-jaleela fit- tawwasuli wal-waseela", with commentary of Dr. Rabi'a bin Hadi 'Umayr al-Mudkhali, professor in the Islamic University of Madinah al-Munawwara, Page 162, para 478:
"But Ahmad ibn Hanbal and other scholars permitted the narration [of hadith] regarding the virtues of good what is not established [as authentic] as long as it is not known that it is a lie." (laakinna Ahmad ibn Hanbal wa ghayruh min al-'ulama jawwazu an yurwa fee fada'il al-'aamal maa lam yu'lam annahu thaabit idha lam yu'lam annahu kadhib.)
And Ibn Taymiyya goes into a full chapter of discussion of this subject from here, Chapter 8 of "al-qaida al-jaleela fit-tawwasuli wal-waseela", where he presents the views of the majority of the 'ulama of Islam and he presents his own views of the subject. So my sunnionline brothers lets take a look closer..
To continue, Ibn Taymiyya says, in para 478:
"and that is the action which is known to be lawful with a shari'ah evidence, and there has been narrated in its virtue a hadith that is not known to be a lie, it is possible that the reward will be true, although none of the Imams have said that it is permissible to consider something required (wâjib) or recommended (mustahabb) by way of a weak hadith, and whoever said so differed from the consensus (ijmâ')."
So here we see that
Ibn Taymiyya is explaining that if there is a hadith, even though it has not been judged to be authentic, if it encourages what is known as a good deed in Islamic shari'ah, something of virtue, a praiseworthy action, or idea, then it is fully acceptable to refer to such a hadith as an encouragement for that deed.
And here
also, Ibn Taymiyya refers to the ijma', the consensus, which is a clear reference to the concept of ijma' of scholars of Islam as being a fully accepted concept and one which he accepts. And this is a clear proof that Ibn Taymiyya, though he considered himself a mujtahid mutlaq, capable of independent reasoning, nevertheless depended on the consensus (ijmâ') of scholars as a proof for the opinions he considered acceptable. And this is the position of Ahl as-Sunnah wal-Jama'at.
Then Ibn Taymiyya continues in para 479:
"And just like it is not permissible to forbid something without a shari'ah evidence, (dalîl shar'î) but if it something is known to be forbidden and a hadith has been narrated in warning the one who commits such an action, and it is not known that it is a lie, it is permissible to narrate it. And it is permissible to narrate it in the manner of encouraging and discouraging (at- tarhîb wat-targhîb) what is not known that it is a lie. but in what is known that Allah has encouraged or discouraged with another evidence besides this [weak] hadith whose authenticity is unknown (majhûl hâluh)."
So from this we see that Ibn Taymiyya is using the weak hadith (ahadîth da'îf), to discourage people from doing an evil deed, as long as this deed is known to be forbidden in the shari'ah. If the deed is forbidden in the shari'ah, it is acceptable to use a hadith whose authenticity is unknown, as long as the hadith is known not to have been an actual lie. This principle is acceptable, in anything that it is known that Allah expressed its forbiddance. Ibn Taymiyya continues to explain this concept in para 480:
"This is like the [situation] of the Isra'iliyyât [stories related by the Jews]. It is permissible to be narrated as long as we know that it is not a lie, for encouraging or discouraging in what we know that Allah has ordered in our law (shar') or forbade in our law (shar')."Here we see that Ibn Taymiyya is not only accepting that the weak are acceptable in the case of encouraging good deeds and disouraging evil ones, but he is showing clearly that he accepted the use of Isra'iliyyât, stories related from the Jews, which many Salafis reject today as unacceptable. And this is verified in the hadith of the Prophet , "narrate from the hadith of Bani Isra'il and there is no harm in doing it."
Ibn Taymiyya continues in para 481:
"As for what has been authenticated as lawful to us, by way of the Isa'iliyyât that has not been proven and no scholar believes in that. And Ahmad ibn Hanbal and the like of the Imams never depend on hadith like this in shari'ah.
para 482:
"So he who transmitted from Ahmad that he used to use weak hadith to support his opinion, that is: neither authentic in grade nor good, he made a mistake about Ahmad." (wa man naqala 'an Ahmad annahu kâna yahtaju bil-hadith ad-da'eef alladhee laysa bi sahih wa lâ hasan faqad ghalata 'alayh).
para 486:
"And this hadith and the like Ahmad calls it weak, and he accepted it as a support of his view." (fa hadha al-hadith wa amthâlah yusammîhi Ahmad da'îfan wa YAHTAJU BIHI).
So we see here that Ibn Taymiyya on one hand is saying, that Ahmad did NOT use WEAK HADITH for establishing what is ordered by a shari'ah ruling AND that whoever said he did was wrong; whereas four paragraphs later he completely reverses himself and says that Ahmad called a hadith WEAK *and* he ACCEPTED IT to encourage someone to keep what is ordered by a shari'ah ruling! This is a disturbing method which we see Ibn Taymiyya employ in numerous other places also. We question here: was it intentional or was he just confused?