It is narrated from Sayyiduna Abu Usayd (may Allah be pleased with him), who was one of the Companions who fought at Badr, that he said: I was sitting with the Prophet (peace and blessings be upon him) when a man of the Ansar came and said: O Messenger of Allah! Is there any way left of honouring my parents after their death? He (peace and blessings be upon him) said: "Yes, there are four things: praying for them and seeking forgiveness for them, fulfilling their pledges after their death, honouring their friends, and maintaining the ties of kinship that you have only through them." The man said: These are the things that remained upon me.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16641