Hadith: Sa‘d ibn ‘Ubādah sought a ruling from the Messenger of Allah (may Allah's peace and blessings be upon him) concerning a vow that was due upon his mother, who had died before she fulfilled it. The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Fulfill it on her behalf."

عن عَبْدُ الله بْنُ عَبَّاسٍ رضي الله عنه قال: «استَفْتَى سعد بن عُبَادَةَ رسول الله في نَذْرٍ كان على أمِّه، تُوُفِّيَتْ قبل أَنْ تقضيَهُ، قال رسول الله صلى الله عليه وسلم : فاقْضِهِ عنها».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported: Sa‘d ibn ‘Ubādah sought a ruling from the Messenger of Allah (may Allah's peace and blessings be upon him) concerning a vow that was due upon his mother, who had died before she fulfilled it. The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Fulfill it on her behalf."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Sa‘d's mother had died, before fulfilling a vow that was due upon her. Her son Sa‘d asked the Prophet (may Allah's peace and blessings be upon him) whether he could fulfill it on her behalf, and the Prophet (may Allah's peace and blessing be upon him) allowed him to do so.