The one who finds fault with some of the people of Ḥadīth and the people of the Jamāʿah, that they speak with excessive, loaded words, they are either finding fault with them based on insufficient knowledge or insufficient understanding. As for the first one [insufficient knowledge], then it is that they use as proofs ḍaʿīf or mawdhūʿ narrations, or āthār [reports from other than the Prophet, ﷺ], that are not fit to be used as proofs. As for the second one [insufficient understanding], then it is that they do not understand the authentic narrations.
[Q]: “Isn’t a mawḍūʿ (fabricated) narration under the category of ḍāʿīf (weak), thus shouldn’t the same leniency be applied? And aren’t the scholars more lenient in dealing with reports that are not marfūʿ’, ie. not narrations of the Prophet (ﷺ)?
[A]: Yes, mawḍūʿ’ (fabrication) is a category of ḍāʿīf; however, the mawḍūʿ’ and ḍāʿīf jiddan (severely weak) narration are not intended when some of the ʿulamāʾ speak about the permissibility of using weak reports for exhortations. It is ḥarām to mention the mawḍūʿ’ narration except to warn against it and inform the people that it is mawḍūʿ’, as the scholars of ḥadīth have mentioned [Tadrīb al-Rāwī, 1/323; al-Bāʿith al-Ḥathīth, 1/238]. As for using mawḍūʿ’ narrations for targhīb and tarḥīb (exhortations), then this is not permissible by way ijmāʿ’ (consensus) of the Muslims. [Tadrīb al-Rāwī, 1/325]
And as for the scholars being more lenient in accepting reports from other than the Prophet (ﷺ), like stories of the Companions and early imāms, then, yes, they are more lenient; however, this is not without limits. They would not use reports from narrators known to be liars! As mentioned, “And as for what has become known as fabricated falsehood,” meaning from the aḥādīth and reports from the Salaf, “Then it is not permissible to glance at it, for verily the lie does not have any benefit.”
This needs more elaboration. The Prophet (ﷺ) said, as found in the two Ṣaḥīḥs:
“Verily the lie forged against me is not like a lie forged against someone else. Whoever lies on me intentionally, then let him take his place in the Fire.”
From the ḥadīth are the following benefits:
Affirmation of a difference between lying on him and lying on someone else.
Affirmation that lying on him carries a severe threat of Hellfire.
Affirmation that lying on someone else is still considered “lying”.
Al-Hāfiẓ ibn Hajr said, “And it is not implied that affirming the mentioned threat for lying on him (ﷺ) makes lying on other than him permissible. Rather, other evidence is used to prove the prohibition of lying on other than him.” [Fat′h al-Bārī, 3/200]
The prohibition of lying is clear from the texts of the Qurʾān and the Sunnah. Allāh has commanded us to be with those who are truthful, and His Messenger (ﷺ) has informed us that one of the attributes of the hypocrite is lying. Thus, it is ḥarām and a major sin, since it is described as a sign of a hypocrite.
Here are some further indications of the correctness of what has been mentioned from the words of Ibn Taymīyyah himself:
1 – Shaykh al-Islām said, “…The one who finds fault with some of the people of Ḥadīth and the people of the Jamāʿah, that they speak with excessive, loaded words, they are either finding fault with them based on insufficient knowledge or insufficient understanding. As for the first one (insufficiant knowledge), then it is that they use as proofs ḍāʿīf or mawḍūʿ’ narrations, or aathaar (reports from other than the Prophet, ﷺ), that are not fit to be used as proofs. As for the second one (insufficient understanding), then it is that they do not understand the authentic narrations…” [al-Fatāwā, 4/23]
Two important benefits that we get from this quote are:
Shaykh al-Islām makes a difference between ḍāʿīf and mawḍūʿ’.
Shaykh al-Islām affirms that there are aathaar that can not be used as proofs.
2 – Shaykh al-Islām said, “…No doubt that this is found with some of them, they use as proofs fabricated narrations in subsidiary and foundational matters, as well as invented aathaar (reports from other than the Prophet, ṣallallāhu ʿalayhi wa-sallam), and stories that are not authentic…” [al-Fatāwī, 18/52]
 Shaykh al-Islām makes a difference between fabricated aḥādīth and fabricated reports and stories from other than the Prophet (ṣallallāhu ʿalayhi wa-sallam).
 Shaykh al-Islām finds fault with both, later referring to them as “evil.”
3 – Shaykh al-Islām said, “As for ‘Az-Zuhd’ of Imām Aḥmad and its likes, then there are no fabricated narrations (of the Prophet ﷺ) or stories (of other than him) in it, like what is in these (books of Abū Nu’aym, Ibn al-Jawzī, and others), for verily he (Imām Aḥmad) did not mention in his books (anything) from anyone who was known to fabricate. Rather, in them there are some that are ḍāʿīf due to the poor memory of their narrators, just as these (fabricated reports) are not in the Musnad. And similarly, there are no mawḍūʿ’ reports that were intentionally fabricated amongst the marfūʿ’ narrations (of the Messenger ﷺ) in them (Imām Aḥmad’s books). However, in them are things known to be mistakes, errors made by their narrators. The likes of these are found in most of the books of Islām, as no book is without errors except the Qurʾān.” [al-Fatāwī, 18/72]
 Shaykh al-Islām affirmed a difference between fabricated narrations ascribed to the Prophet (ṣallallāhu ʿalayhi wa-sallam), and those ascribed to others.
 Shaykh al-Islām clarifies that Imām Aḥmad did not use reports from anyone known to be a fabricator in any of his books, rather the weak narrations he included were only ḍāʿīf and not mawḍūʿ’.
4 – Shaykh al-Islām said, “…And some of it is sheer falsehood (bāṭil qat’an), like what is mentioned in ‘Haqaa’iq at-Tafsīr’, a large amount (of reports) from Ja’far as-Saadiq and others that are from among the fabricated reports (al-aathaar al-mawḍūʿ’ah)…” [al-Fatāwī, 11/42]
Shaykh al-Islām affirms the description ‘mawḍūʿ’ for narrations of other than the Prophet (ṣallallāhu ʿalayhi wa-sallam).
Shaykh al-Islām refers to fabrications that were attributed to Ja’far as-Saadiq, warning against them.
Shaykh al-Islām calls these fabrications ‘sheer falsehood’.
5 – Shaykh al-Islām said, “…And a party of those who claim (to be upon) the Sunnah and the Ḥadīth use as proofs mawḍūʿ’ aḥādīth (of the Prophet ṣallallāhu ʿalayhi wa-sallam) and invented stories (of those other than him), knowing that they are lies…” [al-Fatāwī, 11/339]
6 – Shaykh al-Islām warns against a fabricated story about the severed head of al-Husayn being brought to Yazīd, and other related stories, referring to the one who narrates them as having “no intellect by which to make any distinction between things he says,” and “no understanding of what is being narrated.” [al-Fatāwī, 27/479-483]
And Allāh knows best.