It is narrated from Sayyiduna Abu Tha'labah al-Khushani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, Allah Almighty looks upon His servants with mercy 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 give it up."
Hadith Referenceالمعجم الكبير للطبراني / باب اللام ألف / 18813
It is narrated from Sayyiduna Abu Tha'labah (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) used to fast Sha'ban and Ramadan, joining the two together.
Hadith Referenceالمعجم الكبير للطبراني / باب اللام ألف / 18814