Bukhari

Hadith Commentary:
From this, one can gauge the importance of accepting a permissible invitation, which must indeed be accepted.
This is because the person who refuses does not wish to maintain social relations among Muslims, which is a major pillar of Islam.
Gifts and invitations foster social interaction, and both religious and worldly benefits are dependent upon mutual association and unity. Those who have misunderstood piety (taqwa) to mean that one should remain distant from people and never accept anyone’s invitation—this is not piety, but rather an action contrary to the Sunnah.
However, some simple-minded individuals consider this to be the height of piety—may Allah forgive them and grant them goodness, ameen.

Hadith Commentary:
(1)
In general, the food of a wedding feast (walimah) is not objectionable; rather, when it is characterized by inviting only the wealthy and excluding the poor, then the food of the walimah becomes the worst kind of food. For this reason, Abdullah ibn Mas'ud radi Allahu anhu used to say that when, in any invitation, the wealthy are specifically invited and the poor are left out, we are commanded not to accept such an invitation.

Similarly, it is narrated from Abu Hurairah radi Allahu anhu that he said:
You are committing disobedience in your invitations because you invite those who do not wish to come, and those who wish to come you ignore.
(Fath al-Bari: 9/305)

It is narrated from Ibn Abbas radi Allahu anhuma that he said:
The invitation to a walimah is extremely bad in which those who are already full are invited and the hungry are ignored.
(al-Mu'jam al-Kabir by al-Tabarani: 12/123, no. 12754)

(2)
From this hadith, it is understood that the invitation to a walimah must be accepted, because the term "disobedience" (isyan) is applied to abandoning a command that is obligatory and necessary.
('Umdat al-Qari: 14/134)

Hadith Authentication:
[وأخرجه البخاري 5177، ومسلم 107/1432، من حديث مالك به]

Jurisprudential Understanding
➊ If there are no prohibited acts (munkarat) or frivolity and amusement (lahw wa la‘ib) in the wedding feast (walimah), then it is obligatory (wajib), meaning a duty (fard), to accept the invitation of a brother with correct beliefs (sahih al-‘aqidah).
➋ If there is a legitimate (shar‘i) excuse, then it is permissible (ja’iz) to apologize for not attending the invitation. The Messenger of Allah sallallahu alayhi wa sallam said: «إِذَا دُعِيَ أَحَدُكُمْ فَلْيُجِبْ فَاِنْ كَانَ صَائِمًا فَلْيُصَلِّ وَإِنْ كَانَ مُفْطِرًا فَلْيَطْعَمْ» “When any one of you is invited (to a meal), he should accept it; then if he is fasting, he should supplicate (for the host), and if he is not fasting, he should eat.” [صحيح مسلم : 1431/3520] In another narration, it is stated: “If your brother invites you, then accept it, whether it is a wedding or any other similar invitation.” [صحيح مسلم 1429 [3513]
➌ It is narrated from Sayyiduna Safinah radi Allahu anhu that a man became the guest of Sayyiduna Ali ibn Abi Talib radi Allahu anhu, so he prepared food for him. Then Sayyidah Fatimah radi Allahu anha said: “If only we invite the Messenger of Allah sallallahu alayhi wa sallam so that he may eat with us.” So they invited him, and he came and placed his hand on the door frame, then saw a curtain hanging in one corner of the house. Then he (left without eating and) went back. Sayyidah Fatimah said to Ali radi Allahu anhuma: Go and see why he is going back. (Ali radi Allahu anhu said:) I went and asked: O Messenger of Allah! Why did you go back? The Messenger of Allah sallallahu alayhi wa sallam said: “It is not appropriate for me or for any Prophet to enter a house in which there are images or decorations.”