Sayyidah Umm Salamah (may Allah be pleased with her) said that the Prophet (peace and blessings be upon him) did not fast two months together except Sha'ban and Ramadan. In the hadith of Sufyan, Umm Salamah (may Allah be pleased with her) said: I did not see the Messenger of Allah (peace and blessings be upon him) fasting two consecutive months, except that he (peace and blessings be upon him) would join Sha'ban to Ramadan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7965