Hadith: O Messenger of Allah, my mother has died in a state that she had to observe the fast of a month. Should I complete it on her behalf? Thereupon he said: Would you not pay the debt if your mother had died without paying it? He said: Yes. He said: The debt of Allah is more worthy of being repaid

عن عبد الله بن عباس رضي الله عنهما قال: «جاء رجل إلى النبي صلى الله عليه وسلم فقال: يا رسول الله، إن أمي ماتت وعليها صوم شهر. أَفَأَقْضِيهِ عنها؟ فقال: لو كان على أمك دَيْنٌ أَكُنْتَ قَاضِيَهُ عنها؟ قال: نعم. قال: فَدَيْنُ اللهِ أَحَقُّ أن يُقْضَى ». وفي رواية: «جاءت امرأة إلى رسول الله صلى الله عليه وسلم فقالت: يا رسول الله، إن أمي ماتت وعليها صوم نذر. أفأصوم عنها؟ فقال: أرأيت لو كان على أمك دَيْنٌ فَقَضَيْتِيهِ ، أكان ذلك يُؤَدِّي عنها؟ فقالت: نعم. قال: فَصُومِي عن أمك».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported that a man came to the Prophet (may Allah's peace and blessings be upon him) and said: "O Messenger of Allah, my mother has died in a state that she had to observe the fast of a month. Should I complete it on her behalf?" Thereupon he said: "Would you not pay the debt if your mother had died without paying it?" He said: 'Yes.' He said: "The debt of Allah is more worthy of being repaid." Another narration states that a woman came to the Prophet (may Allah's peace and blessings be upon him) and said: "O Messenger of Allah, my mother passed away and she took an oath to fast, should I fast on her behalf?" He said: "Tell me, if there was a debt due on your mother and you settled it, would it be enough?" She replied: 'Yes.' He said: "So, fast on behalf of your mother."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

This Hadīth has two narrations, and it is clear from the context that they were two occasions. The first narration tells about a man who came to the Prophet (may Allah's peace and blessings be upon him) and told him that his mother passed away without fasting one month that was due on her, so should he fast it on her behalf? The second narration is about a woman who came to the Prophet (may Allah's peace and blessings be upon him) and told him that her mother passed away without fasting a month that she vowed to fast, so should she fast on her behalf? In both cases, the Prophet told them that they should fast on behalf of their parents. The Prophet made an analogy to make the judgment clear by asking if their parents were indebted to a human being, would they pay their debts? They answered in the affirmative. The Prophet (may Allah's peace and blessings be upon him) told them that this fasting is a debt their parents owe to Allah. Since the debt to humans can be settled, the debt owed to Allah deserves more to be settled.