Fasting Al-Ayyam Al-Beed in Sha`ban
Question

I have developed the habit of fasting al-ayyaam al-beed each month, praise be to Allah, but this month I did not fast. When I wanted to fast I was told that it is not permissible and is bid’ah. I fasted on the 1st of the month, then I fasted on the 19th of Sha’baan, and if Allah wills I will fast tomorrow, then I will have fasted three days. What is the ruling? And what is the ruling on fasting a great deal during the month of Sha’baan?
Praise be to Allah.Speaking about Allah without knowledge is not allowed in Islam
Allah has forbidden us to speak about Him without knowledge, and He mentions that alongside shirk and major sins. Allah says (interpretation of the meaning): 
“Say (O Muhammad): (But) the things that my Lord has indeed forbidden are Al-Fawahish (great evil sins and every kind of unlawful sexual intercourse) whether committed openly or secretly, sins (of all kinds), unrighteous oppression, joining partners (in worship) with Allah for which He has given no authority, and saying things about Allah of which you have no knowledge.” [al-A'raf 7:33]
An example of speaking about Allah with no knowledge is what is mentioned in the question, that some people say it is a bid’ah (an innovation) to fast three days in Sha’ban in the manner described in the question. 
Al-ayyam al-beed fasting:  When?
it is mustahabb to fast three days each month , and it is best to fast al-ayyam al-beed , which are the 13th, 14th and 15th of the month. 
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: My close friend [the Prophet (peace and blessings of Allah be upon him)] advised me to do three things which I will not give up until I die: fasting three days of each month, praying Duha and going to sleep after Witr. (Narrated by al-Bukhari (1124); Muslim (721) 
It was narrated that ‘Abd-Allah ibn ‘Amr ibn al-‘As (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: “It is sufficient for you to fast three days every month, because for every good deed you will have (the reward of) ten like it, so that will be like fasting for a lifetime.” (Narrated by al-Bukhari (1874); Muslim (1159)
It was narrated that Abu Dharr said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: “If you fast any part of the month, then fast the thirteenth, fourteenth and fifteenth.” (Narrated by al-Tirmidhi (761); al-Nasai (2424)
This hadith was classed as hasan by al-Tirmidhi, and al-Albani agreed with him in Irwa al-Ghalil, 947. 
Shaykh Muhammad ibn Salih al-‘Uthaymin (may Allah have mercy on him) was asked: 
It was narrated in the hadith that the Prophet (peace and blessings of Allah be upon him) advised Abu Hurayrah (may Allah be pleased with him) to fast three days of each month. When should these fasts be observed, and should they be one after the other? 
He replied: 
It is permissible to fast these days one after the other or separately, and they may be at the beginning of the month, or in the middle, or at the end. The matter is broad in scope, praise be to Allah, because the Messenger of Allah (peace and blessings of Allah be upon him) did not state any specific days. ‘Aishah (may Allah be pleased with her) was asked: Did the Messenger of Allah (peace and blessings of Allah be upon him) fast three days of each month? She said: Yes. It was asked: In which part of the month did he fast? She said: He would not bother in which part of the month he would fast. Narrated by Muslim, (1160). But the 13
th, 14th and 15th are better, because these are al-ayyam al-beed.” (Majmu’ Fatawa al-Shaykh Ibn ‘Uthaymin, 10/ question no. 376) 
Al-Ayyam al-Beed fasting in Sha’ban: Permissible?
 Perhaps the one who told you not to fast on those days in this month (Sha’ban) said that because he knew that the Prophet (peace and blessings of Allah be upon him) forbade fasting in the latter part of Sha’ban. 
In the answer to a question about making up missed Ramadan fasts in the second half of Sha’ban , we have explained that this prohibition applies to one who is starting to fast in the latter part of Sha’ban and does not have a prior habit of fasting. 
But if a person starts to fast in the first half of Sha’baan and continues to fast in the latter half, or he has a prior habit of fasting, then there is nothing wrong with him fasting in the latter half of Sha’baan, such as one who has the habit of fasting three days each month, or fasting on Mondays and Thursdays. 
Based on this, there is nothing wrong with you fasting three days in Sha’baan, even if some of those happen to be in the latter half of the month. 
Fasting in Sha’ban: Recommended?
There is nothing wrong with fasting a great deal in Sha’ban , indeed that is Sunnah. The Prophet (peace and blessings of Allah be upon him) used to fast a great deal during this month. 
It was narrated that ‘Aishah (may Allah be pleased with her) said: The Messenger of Allah (peace and blessings of Allah be upon him) used to fast until we thought he would not break his fast, and he used not to fast until we thought he would not fast. I never saw the Messenger of Allah (peace and blessings of Allah be upon him) fast a complete month except Ramadan and I never saw him fast more than in Sha’ban. (Narrated by al-Bukhari, 1868; Muslim, 1165) 
Abu Salamah narrated that ‘Aishah (may Allah be pleased with her) said: The Prophet (peace and blessings of Allah be upon him) did not fast in any month more than Sha’ban, and he used to fast all of Sha’ban and he used to say: “Do as much of good deeds as you can, for Allah does not get tired (of giving reward) until you get tired.” And the most beloved of prayers to the Prophet (peace and blessings of Allah be upon him) was that in which one persists even if it is little. When he offered a prayer he would persist in doing so. (Narrated by al-Bukhari, 1869; Muslim, 782) 
And Allah knows best.