It was narrated from Ibn 'Abbas' that :
Sa'd bin 'Ubadah asked the Messenger of Allah (ﷺ) about a vow which his mother had made, but she had died without fulfilling it. The Messenger of Allah (ﷺ) said: "Fulfill it on her behalf ." (FR)
English reference (FR): Vol. 3, Book 11, Hadith 2132 (FR)
Arabic reference (FR): Book 11, Hadith 2214 (FR)