Ummi Salaah Radiyallahu 'Anha reports:
"I did not observe Rasulullah Sallallahu 'Alayhi Wasallam fast for two consecutive months, besides the month of Sha'baan and Ramadhaan". (ID)
English reference (ID): Book 42, Hadith 284 (ID)
Arabic reference (ID): Book 43, Hadith 301 (ID)