A weak hadith that is circulated on this issue states that it was narrated from Abu Moosa al-Ash’ari that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Allah looks down on the night of the fifteenth of Shaaban and forgives all his creation except a mushrik or one who harbors hatred against the Muslims.” Narrated by Ibn Maajah, 1390. (The “one who harbors hatred against the Muslims” means one who has enmity towards a Muslim brother.)
There is some scholarly difference of opinion as to the soundness of such ahaadeeth. There is no saheeh hadeeth concerning the virtue of the night of the fifteenth of Shaaban, especially as it relates to Allah descending to forgive sins. For such ahadith, the isnaads (chain of narrations) are not free of some weakness, and some of them are very weak. Many other scholars have attributed this hadith to be as weak because of the weakness of the chain of narrators.
The scholars have maintained that Allah’s descending to the first heaven does not only happen on the night of the fifteenth of Shaaban, rather it is proven in al-Saheehayn and elsewhere that Allah descends to the first heaven every night, in the last third of the night. The night of the fifteenth of Shaaban is included in this general meaning.
Hence, when ‘Abd-Allaah ibn al-Mubaarak was asked about the descent of Allah on the night of the fifteenth of Shaaban, he said to the one who asked him: “…The night of the fifteenth?! He descends every night!” (Narrated by Abu ‘Uthmaan al-Saabooni in I’tiqaad Ahl al-Sunnah, no. 92.)