It is narrated from Sayyiduna Abu Tha'labah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Allah Almighty turns to His servants on the fifteenth night of Sha'ban; He forgives the believers, gives respite to the disbelievers, and leaves the people of rancour with their rancour until they call upon Him (Allah)."
Hadith Referenceالمعجم الكبير للطبراني / باب الياء / 18882