Rasool salal laho aliehi wasalam is the last messenger of Allah swt and his words and sayings besides Quran are authority for us.Allah swt has clearly stated in Holy Quran:
Indeed in the Messenger of Allâh (Muhammad SAW) you have a good example to follow for him who hopes in (the Meeting with) Allâh and the Last Day and remembers Allâh much. (Al-Ahzab 33:21)
On another instance Allah swt says:
He who obeys the Messenger (Muhammad SAW), has indeed obeyed Allâh, but he who turns away, then we have not sent you (O Muhammad SAW) as a watcher over them. (An-Nisa 4:80)
These verses of Quran clearly indicate that sayings of Rasool salal laho aliehi wasalam are authority in matters of deen .
Now we should look in to the matter of Shab e baraat in light of Quran and Sunnah .
It is an obvious fact that Quran was revealed in month of Ramadan from Loh e Mehfooz to first heavens in Night of Qadar .This reality has been elaborated in Holy Quran in following way:
Verily! We have sent it (this Qur'ân) down in the night of Al-Qadr (Decree) (Al-Qadr 97:1)
And Allah swt has also made it clear in the Holy Quran that in this night decisions are made.
Allah swt says:
Therein descend the angels and the Rûh [Jibrael (Gabriel)] by Allâh's Permission with all Decrees, (Al-Qadr 97:4)
So it is clear that decisions about human beings are made in night of Decree which is the night of Ramadan .
Now Some of the people have innovated the idea of another night and named it has lailah e mubarakah or blessed night and claimed that it is night of 15th Shaban. They quote the verse of Surah Dukhan about it that in this night decisions are made so it should be celebrated like night of Qadar in Ramadan.
When we look into the text of Surah Dukhan it appears as follows:
We sent it (this Qur'ân) down on a blessed night. Therein (that night) is decreed every matter of ordainments. Amran (i.e. a Command or this Qur'an or the Decree of every matter) from Us. Verily, We are ever sending (the Messengers), (Ad-Dukhan 44:3-5)
Now it is very clear that all the matters are decided in blessed night.Now we have to resolve the reality of blessed night in the light of verses of Holy Quran .In surah Qadar Allah says that it was revealed in night of decree and in this night all matters are decided.On the other instance in Surah Dukhan Allah says that it was revealed in blessed night in which all matters are decided .
It is clear from these two surahs that blessed night and night of decree are the names of same night like Muhammad and Ahmad salal laho aliehi wasalam are the name of same prophet .There is no difference between these two nights .So any one who want to seek the blessings of Allah in blessed night he should search it in last five odd nights of Ramadan .
When we consult hadiths on this particular issue we come to know that there is no authentic hadith that highlights the significance of this particular night.There are some weak hadiths but they cannot be taken as authority on this issue.
Prophet salal laho aliehi wasalam used to fast on 13th 14th and 15th of every month so any one who fasts on these nights he should also fast on 13th 14th and 15th of Shabaan.
But as far as the fasting on 15th of shabaan only is concerned it is not proved from any authentic hadith .So what we have to do is to follow Quran and hadith in all matters of life .May Allah guide us all to right path ameen.