Sayyidah Aishah and Sayyidah Umm Salamah (may Allah be pleased with them both) both said: "The Prophet (peace and blessings be upon him) did not fast any month other than Ramadan except Sha'ban, for he (peace and blessings be upon him) used to join it to Ramadan, and so they would become two consecutive months. And he (peace and blessings be upon him) would sometimes fast so much that we would say he will not stop fasting, and sometimes refrain so much that we would say he will not fast."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19832