It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) passed by a grave in which a deceased person had been buried, and he (peace and blessings be upon him) said: "Whose grave is this?" They said: It is the grave of so-and-so. He (peace and blessings be upon him) said: "Why did you not inform me?" The narrator said: They regarded him as insignificant and thought little of him. Then he (peace and blessings be upon him) offered the funeral prayer over him after he had been buried, and he (peace and blessings be upon him) said: "These graves are filled with darkness, and Allah Almighty illuminates them through my prayer."
Thabit also narrated it from Abu Rafi' from Abu Hurayrah, and it is mahfuz (preserved) through both routes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 7011