For those searching, this may be the quote you were referring to. This is related by St. Alphonsus Liguori here on p. 573: https://archive.org/details/alphonsusworks15liguuoft/page/572/mode/2up
Father Seraphine Razzi relates that in a city in Italy there was a married lady of noble rank who was reputed a saint. On her deathbed she received all the sacraments, and died with a high reputation for sanctity. After death her daughter, who always recommended to God the soul of her mother, heard one day, while she was at prayer, a great noise at the door. She turned round, and saw a horrible figure all on fire, and exhaling a great stench. At this sight she was so much terrified, that she was on the point of throwing herself out of the window; but she heard a voice saying: "Stop, stop, my daughter: I am your unhappy mother, who was considered a saint; but for some sins committed with your father, which I was ashamed ever to confess, God has condemned me to hell. Do not pray to God for me any more; for you only increase my pains." She then began to howl, and disappeared.