A Jewish woman told Aisha about punishment in the grave, saying may Allah protect her from it. Aisha asked the Prophet Muhammad about it, and he confirmed there is punishment in the grave. After that, Aisha noticed the Prophet seeking refuge from the punishment of the grave in every prayer.

2.  Narrated Masruq: 'Aisha said that a Jewess came to her and
mentioned the punishment in the grave, saying to her, "May
Allah protect you from the punishment of the grave." 'Aisha
then asked Allah's Apostle about the punishment of the
grave. He said, "Yes, (there is) punishment in the grave."
'Aisha added, "After that I never saw Allah's Apostle but
seeking refuge with Allah from the punishment in the grave in
every prayer he prayed."
(Sahih Al-Bukhari, Volum # 2, Book 23, Hadith # 454)