How sound is the Hadith: "The Messenger of Allah, peace and blessings be upon him, used to fast three days of every month, and sometimes he would defer that until the fast of the year accumulated upon him, so he would fast Sha'ban"?

The Prophet, peace and blessings be upon him, used to fast frequently in Sha'ban, to the extent that 'Aisha, may Allah be pleased with her, said that he never completed the fast of an entire month except Ramadan, and she never saw him fast more in any month than in Sha'ban. The Prophet, peace and blessings be upon him, explained the reason for this, saying that it is a month people overlook, coming between Rajab and Ramadan, and in it, deeds are raised to the Lord of the worlds. So, he loved for his deeds to be raised while he was fasting.