What is the ruling on one who vows to fast a month if he passes his exam?

Question:
May Allah reward you, Honorable Shaykh. A student took the exam but was unable to complete her answers before time ran out, and she thought that she… Yes. He says: A student took the exam and was unable to complete her answers because time ran out, and she thought she would not pass. So her classmate said to her, “If you pass, you will fast a month?” She said, “Yes.” Then she passed, and she is now asking what is obligatory for her?

Answer:
If she vowed that if she succeeded she would fast, and she did succeed, she is not required to fast, because this is a vow of obedience. Indeed, Allah the Exalted said: and He said: "And let them fulfill their vows." The Prophet (peace and blessings be upon him) said: [It is] conditional upon success. If this happens, it is a blessing from Allah; this is a blessing from Allah. Success is a blessing from Allah. She vowed that if she received this blessing, she would fast in gratitude to Allah the Mighty and Majestic. Therefore, she is required to fulfill what she vowed and fast this month.