Musnad Ahmad 220:1

Ibn Abi ‘Adi narrated to us, from Dawud ibn Abi Hind, from al-Sha‘bi, from ‘Alqamah, from Salamah ibn Yazid al-Ju‘fi, who said: I and my brother went to the Messenger of Allah, may Allah bless him and grant him peace. We said: "O Messenger of Allah, our mother, Mulaykah, used to uphold the ties of kinship, host the guest, and do such and such, and such and such, but she died during the Jahiliyyah (pre-Islamic period). Will that be of any benefit to her?" He said: "No." We said: "She buried alive a sister of ours during the Jahiliyyah. Will that be of any benefit to her?" He said: "The one who buries alive and the one buried alive are in the Fire, unless the one who buries alive reaches Islam, and Allah pardons her."