The best act of Birr
The Messenger of Allah (sal Allahu alaihi wa sallam) said: “The best act of Birr is for a man to uphold ties with the friends of his father.”
This Hadith shows the virtue of one keeping ties with their fathers friends after their death whereby the Messenger (sal Allahu alaihi wa sallam) mentioned that it is from the greatest acts of Birr (obedience to ones parents) for a person to keep ties with their fathers friends after their father has died.
They can do this by treating them well and being generous towards them and this act is considered Birr towards the father because the father is the reason that the person is keeping ties with his friends.
Imam An-Nawawi mentioned that this also applies to the friends of the mother, grandparents siblings, husband and wife as mentioned in the Hadith whereby the Messenger (sal Allahu alaihi wa sallam) kept ties with the friends of Khadeejah (radi Allahu anha) after her death.
And Allah Knows Best!