Al-Nawawi said: In the words, “He used to fast all of Sha’baan, he used to fast Sha’baan except a few days” the second phrase explains the first, and indicates that the word “all” means “most of”.
This hadith indicates that it is permissible to fast after halfway through Sha’baan, but only for those who are continuing after fasting before halfway through the month. The Shaafa’is followed all of these ahaadith and said:
It is not permissible to fast after halfway through Sha’baan except for those who have a habitual pattern of fasting, or who are continuing after fasting before halfway through the month.
According to most of the scholars, the prohibition here means that it is haraam.
See al-Majmoo’, 6/399-400; Fath al-Baari, 4/129
Some, such as al-Ruyaani, suggested that the prohibition here is to be understood as meaning that it is makrooh, not haraam.
Al-Nawawi (may Allah have mercy on him) said in Riyaadh al-Saaliheen (p. 412):
“Chapter on the prohibition on anticipating Ramadan by fasting after halfway through Sha’baan, except for one who is continuing after fasting before halfway through the month or who has a regular pattern of fasting such as fasting on Mondays and Thursdays”.
The majority of scholars are of the view that the hadith which forbids fasting after halfway through Sha’baan is da’eef (weak), and based on that they said that it is not makrooh to fast after halfway through Sha’baan.
Al-Haafiz said: The majority of scholars said that it is permissible to observe voluntary fasts after halfway through Sha’baan and they regarded the hadith concerning that as da’eef. Ahmad and Ibn Ma’een said that it is munkar. (From Fath al-Baari). Among those who classed it as da’eef were al-Bayhaqi and al-Tahhaawi.
Ibn Qudaamah said in al-Mughni that Imam Ahmad said concerning this hadith:
It is not sound. We asked ‘Abd al-Rahmaan ibn Mahdi about it and he did not class it as saheeh, and he did not narrate it to me. He used to avoid talking about this hadith. Ahmad said: al-‘Ala’ is thiqah and none of his ahaadith are regarded as munkar apart from this one.
The al-‘Ala’ referred to here is al-‘Ala’ ibn ‘Abd al-Rahmaan who narrated this hadith from his father from Abu Hurairah (may Allah be pleased with him).
Ibn al-Qayyim (may Allah have mercy on him) responded in Tahdheeb al-Sunan to those who classed this hadith as da’eef and said that this hadith is sahih according to the conditions of Muslim. Even though al-‘Ala’ is the only one who narrated this hadith, that is not regarded as detrimental to the hadith, because al-‘Ala’ is thiqah; in his Sahih, Muslim narrated a number of ahaadith from him, from his father from Abu Hurairah (may Allah be pleased with him). Many Sunnahs are narrated from the Prophet (peace and blessings of Allah be upon him) only through one person who is thiqah, but they have been accepted and followed by the ummah.
Then he said:
With regard to those who think that there is a contradiction between this hadith and the ahaadith which speak of fasting in Sha’baan, there is no contradiction. Those ahaadith speak of fasting half of it along with the previous half, and of habitual fasting during the second half of the month, whereas the hadith of al-‘Ala’ speaks of the prohibition on fasting deliberately only after the month is halfway over, not about fasts that a person observes regularly or that are a continuation after fasting during the first part of the month.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked about the hadith which says that fasting after halfway through Sha’baan is not allowed. He said:
This is a sahih hadith as Shaykh Naasir al-Deen al-Albaani said. What is meant is that it is not allowed to start fasting after halfway through the month. But if a person fasts most or all of the month, then he is following the Sunnah.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 15/385).
Shaykh Ibn ‘Uthaymeen said in his commentary on Riyaadh al-Saaliheen (3/394):
Even if the hadith is sahih, the prohibition in it does not mean that this is haraam, rather it is simply makrooh, as some of the scholars have understood it to mean. But whoever has the habit of fasting regularly should fast, even if it is after halfway through Sha’baan.
It is not allowed to fast during the second half of Sha’baan, and that is either makrooh or haraam, except for the one who has the habit of fasting regularly or who is continuing after fasting during the first half of Sha’baan. And Allah knows best.
The reason for this prohibition is that continually fasting may make a person too weak to fast in Ramadan.
If it is said that if he fasts from the beginning of the month he will become even weaker, the response is that whoever fasts from the beginning of Sha’baan will have gotten used to fasting so it will be less difficult for him to fast.
Al-Qaari said: The prohibition here means that it is disliked, as a mercy to this ummah lest they become too weak to fulfil their duty of fasting during
Ramadan in an energetic fashion. But those who fast all of Sha’baan will become used to fasting so it will not be difficult for them.
And Allah knows best.