that he consulted the Prophet (ﷺ) about a vow that had been made by his mother who died without fulfilling it. The Prophet (ﷺ) gave his verdict that he should fulfill it on her behalf. The verdict became Sunna (i.e. the Prophet's tradition).
Sahih al-Bukhari Book 83, Hadith 75
← Previous Hadith `Umar said "O Allah's Messenger (ﷺ)! I vowed to perform I`tikaf for one night in Al-Masjid-al-Haram, during the Pre-Islamic Period of ignorance (before embracing Islam). "The Prophet (ﷺ) said, "Fulfill your vow." Ibn `Umar said to the lady, "Pray on her behalf." Ibn `Abbas said the same.

Sahih al-Bukhari Book 83, Hadith 74 Next Hadith → A man came to the Prophet (ﷺ) and said to him, "My sister vowed to perform the Hajj, but she died (before fulfilling it)." The Prophet (ﷺ) said, "Would you not have paid her debts if she had any?" The man said, "Yes." The Prophet (ﷺ) said, "So pay Allah's Rights, as He is more entitled to receive His rights."

Sahih al-Bukhari Book 83, Hadith 76