It is not allowed to precede Ramadan by fasting a day or two before it begins
Abu Hurayrah (radiyallāhu ‘anhu) narrated that Allah’s Messenger (salallāhu ‘alaihi wasallam) said: “Do not precede Ramadan by fasting a day or two days before it except for a person who is in the habit of fasting, so let him fast.”
This hadeeth contains a prohibition from the Prophet (salallāhu ‘alaihi wasallam) from fasting Ramadan before it actually begins such a person saying, “I want extra reward so I’ll fast a day or two before Ramadan, and continue fasting till the end of the month.” This is forbidden because it is increasing upon what Allah (the Most High) has obligated, and is, therefore, an innovation. An innovation (bid’ah) is to introduce something into the Religion which Allah has not legislated. So, it is not permitted to fast a day or two days before Ramadan upon the thought that these days are linked to Ramadan as that would be an innovation connected to a legislated act.
The exception to this is “a person who is in the habit of fasting”. This would be, for example, a person who is in the habit of fasting three days every month, or the one who regularly fasts Monday and Thursday, or the one who fasts the fasting of the Prophet Dawūd (‘alaihis-salām), or that one who still has fasts to make up from the previous Ramadan or the one who must fast to fulfil an oath that he has to fast till the end of the month of Sha’bān. In these scenarios, there is no harm in his fasting because he has not opposed the hadīth and has fasted for a purpose other than what the narration forbids.
So, it is not permissible to increase acts of worship which Allah (the Most High) has limited―it is not allowed, for example, to increase a four-rak’ah prayer to five or six rak’ahs, or to make Fajr prayer into three or four rak’ahs. And if a person argues, “I only want good,” we say, “No, this increase is evil, not good―it is not permitted for a person to increase upon the Sharī’ah of Allah.” So, it is not allowed to add anything to the fasting month of Ramadan and the month begins with a sighting with the eyes and if that is not possible due to cloud-cover then with the completion of thirty days of the month of Sha’bān.
‘Ammār bin Yāsir (radiyallāhu ‘anhu) said: “Whoever fasts the day of wherein there is doubt has disobeyed Abul-Qāsim (salallāhu ‘alaihi wasallam).” The day of doubt is the thirtieth day of Sha’bān . If the new (crescent) moon was sighted on the twenty-ninth of Sha’bān, then the next day would be the first day of Ramadan. But if the moon was not sighted, or if there was cloud-cover that prevented sight of the sky, then the following day is the thirtieth of Sha’bān. It is for that reason it is called the day of doubt―because it carries the possibility that it may be the first of Ramadān. It is not allowed to fast on this day in accordance to this hadith of ‘Ammār bin Yāsir (radiyallāhu ‘anhu), “Whoever fasts the day of wherein there is doubt has disobeyed Abul-Qāsim (salallāhu ‘alaihi wasallam).” This command in this narration takes the ruling of coming from the Messenger (salallāhu ‘alaihi wasallam).
Abul-Qāsim is Allah’s Messenger (salallāhu ‘alaihi wasallam)―and Al-Qāsim was his eldest son, Khadeejah (radiyallāhu ‘anhā) gave birth to him and he died in Makkah. So, the Prophet’s kunyah  (the father of Al-Qāsim), was due to this son and it is a term of respect and honour.
 Al-Bukhāri (no. 1914), Muslim (no. 1082).
 Known in Sharī’ah terms as al-bid’ah al-idāfiyyah.
 The Prophet (salallāhu ‘alaihi wasallam) said: “O Abdullāh bin ‘Amr, you fast continually and stand in prayer through the night. If you carry on, you will harm your sight and lose it. There is no reward of fasting for the one who fasts continually. Fasting three days of the month is like fasting the whole month.” ‘Abdullāh (radiyallāhu ‘anhu) said: “But I can do more.” So, the Prophet (salallāhu ‘alaihi wasallam) said: “Then fast the fasting of Dawūd (alaihis-salām)―he used to fast one day and leave it off the following day and so on, and he would never turn away from an encounter [in Allah’s cause].” (Muslim, no. 1159)
 Abu Dawūd (no. 2334), Ibn Mājah (no. 1645), At-Tirmidhi (no. 686), Al-Bukhāri (with a disconnected chain before hadith no. 1906); authenticated by Ibn Khuzaymah (no. 1914), Ibn Hibbān (nos. 3585, 3595, 3596), and Al-Albāni in Irwā Al-Ghalīl.
 The month that precedes Ramadan.
 i.e. his appellation and title, Abul-Qāsim.