It is obligatory on you to fast the month that you made the vow to observe Sawm in if you were successful, as this vow
is a conditional vow of obedience. When the condition of your vow is realized, it becomes obligatory on you to fulfill your vow, because the Prophet (peace be upon him) said,
“Anyone who vows to obey Allah, should obey Him.”
(Part No. 23; Page No. 322) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.