The hadith concerning the virtue of fasting in Sha’ban alluded to in Fasting in Sha’ban, The Neglected Month is related by Abu al-Ghusn Thabit ibn Qays: from Abu Sa’id al-Maqburi: from Usamah ibn Zayd who states:
«لم يكن يصوم من الشهر ما يصوم من شعبان»
“He (the Prophet صلى الله عليه وسلم) did not used to fast any month as much as he would fast Sha’ban…”
So Usamah asked him:
لم أرك تصوم من الشهر ما تصوم من شعبان
“I have not seen you fasting any month as you fast Sha’ban.”
«ذاك شهر يغفل الناس عنه بين رجب ورمضان، وهو شهر ترفع فيه الأعمال إلى رب العالمين عز وجل فأحب أن يرفع عملي وأنا صائم»
“This is a month that people neglect, between Rajab and Ramadan, and it is a month in which the deeds are raised up to the Lord of the Worlds, so I love that my deeds should be raised up while I am fasting.”
This hadith is reported by Ahmad and an-Nasa’i. Shaykh al-Albani deemed it hasan and the author of al-Furu’, ibn Muflih, stated that its isnad is jayyid (good).
However, its isnad contains one Thabit ibn Qays Abu al-Ghusn whose status is disputed. He has been judged thiqah (trustworthy) by Ahmad. An-Nasa’i said, “Laysa bihi ba’s (there is no harm in him).” Ibn Ma’in said about him, “Laysa bihi ba’s (there is no harm in him),” and in another narration, “His hadiths are not that (strong), and he is salih (acceptable).” In another narration still mentioned by ibn Hibban, ibn Ma’in called him da’if (weak). Both ibn Sa’d and ibn Hibban mentioned that he only narrates a few hadiths. Ibn Hibban stated about him, “He narrates little and errs often, he is not to be used as a proof in narrations for which he is not corroborated (by another narrator).”
In his book, al-Kamil Fi ad-Du’afa’, it is ibn ‘Adi’s habit to begin the biography of every narrator by mentioning what statements have reached him concerning that narrator from the Imams of al-Jarh wa at-Ta’dil. Afterwards, he cites some of that narrator’s hadiths as examples to illustrate the reliability, or lack thereof, of the narrator in question. Finally, he states his conclusion.
Ibn ‘Adi, after citing some of the statements of Ahmad and ibn Ma’in, he related a number of the hadiths of Thabit ibn Qays, amongst them our current hadith. He concludes saying, “He is amongst those whose hadiths are to be written.” This general statement concerning Thabit ibn Qays does not necessarily mean that his hadiths are acceptable absolutely, rather, it indicates that any weakness he may have does not reach a level that his hadiths must be abandoned altogether. At the same time, ibn ‘Adi does not explicitly state that his narrations should not be used as proof as ibn Hibban did.
It is well known that ibn Hibban is very severe in criticizing narrators for their mistakes and this case would be an illustration of that.
If we look to al-Hafiz ibn Hajar, we find in his work, at-Taqrib, that he reached the following conclusion concerning him, “Saduq yahim (a truthful narrator who errs).” The term saduq is typically used concerning narrators whose narrations are considered hasan, however ibn Hajar indicated in the introduction of at-Taqrib that when he says about a narrator Saduq yahim or something similar that he means he is weak, although he is amongst the highest level of weak narrators. It seems that this is a fair and balanced judgment concerning him.
If we consider the judgment of Ahmad, it is possible that he was referring to his trustworthiness in character and not his reliability as a narrator as is sometimes the case when the Imams of al-Jarh wa at-Ta’dil call a narrator thiqah. From the statements of ibn Ma’in, an-Nasa’i, ibn Hibban, and ibn Ma’in, we can gather that clearly there is at least some reservation about the accuracy of his narrations. Furthermore, given that he does not narrate a great deal of hadiths, it is simply not possible to make a definitive conclusion concerning his precision and accuracy.
Hence, it can be said, that this hadith is not definitively corroborated to be authentic. This does not mean that it is categorically false, simply that it cannot be confirmed as authentic, and Allah (تعالى) knows best.