If the Prophet (peace and blessings of Allaah be upon him) taught us these acts of worship, then where is the evidence (daleel) for singling out the day of the Mi’raaj or any other day for fasting?
If there were any evidence that it is prescribed to fast on these two days, no one would be able to say that fasting on these days is bid’ah. But what is apparent is that those who say this mean that fasting is an act of worship in general terms, so that if he fasts he has done an act of worship for which he will be rewarded, so long as it is not on one of the days when fasting is not allowed, such as on Eid. This would be correct if the person who is fasting did not single out a day which he believes is a day of virtue, such as the day of the Mi’raaj or the day of Shab-e-baraath.
What makes this the matter of bid’ah is the fact that one is singling out these days. If there was any virtue in fasting these two days, the Prophet (peace and blessings of Allaah be upon him) would have fasted them, and he would have urged us to fast them.

It is known that the Companions of the Messenger of Allaah were more keen to do good than we are. if they had known that there was any virtue in fasting these two days, they would have fasted them. Since we find no reports to that effect from them, we know that this is an innovated bid’ah, and the Prophet (peace and blessings of Allaah be upon him) said:

“Whoever does an action that is not a part of this matter of ours (Islam) will have it rejected,”

i.e. it will be thrown back on the one who does it. Fasting these two days is an action which we find no report of the Prophet (peace and blessings of Allaah be upon him) enjoining, so it is to be rejected.