Musnad Ahmad 1153:1024

Waki' narrated to us, Ubaydullah ibn al-Walid narrated to us, from Abdullah ibn Ubayd ibn Umayr, from Aisha, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him, about sudden death. He said: "It is a relief for the believer, and a seizure of regret for the wicked person."