Sayyiduna Abu Usayd al-Sa'idi (may Allah be pleased with him) said: A man from Banu Sa'idah came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah! My parents have died. Is there any kindness towards them still remaining that I can do for them after their death?" He (peace and blessings be upon him) said: "Yes! Four things: praying the funeral prayer over them, seeking forgiveness for them, carrying out their pledges after their death, honouring their friends, and maintaining the ties of kinship which you have only through them." The man said: "O Messenger of Allah! How much this is, and how good!" He (peace and blessings be upon him) said: "Then keep acting upon it, for it will reach them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6893