Riyad-us-Saliheen 583
Buraidah (May Allah be pleased with him) reported: The Prophet (PBUH) used to teach them (i.e., the Companions) whenever they came out to visit the graveyard to say: "As-salamu 'alaikum ahlad-diyyari minal-Mu'minina wal-Muslimina, wa inna in sha' Allahu bikum lahiqun. As'alul-laha lana wa-lakumul- 'afiyyah (May you be safe, O inmates of the abodes of the believers and the Muslims, and if Allah pleases, we shall follow you, we pray to Allah for well-being for ourselves and for you)."
[Muslim]
This Hadith has a provision for a believer to visit the graveyard and to pray for his dead relatives, because the prayer of the living benefits the departed souls.