Musannaf Ibn Abi Shayba 33:983

Yazid ibn Harun narrated to us, he said: Abu Muhammad al-Asadi narrated to us, from al-Hasan, who said: Whoever enters the graveyards and says: "O Allah, Lord of the decayed bodies and the rotten bones that departed from the world while they were believers in You, admit upon them a spirit from You and peace," forgiveness will be sought for him by every believer who has died since Allah created Adam.