[Q]: The saying of the Prophet (صلى الله عليه وسلم) in the ḥadīth: “If one of your brothers invites you, then answer his invitation whether it be a wedding or other than it,”1 what is meant by ‘your brother’ here? Should we answer the invitation of a religious innovator? Or the wedding invitation of a disbeliever who is among one’s close family or friends while that wedding consists of that which is shirk. Should one answer such an invitation or leave it?
[A]: If the gathering consists of that which is prohibited according to Islamic legislation, then he should not answer this invitation, whether the inviter is a Muslim or a disbeliever. It is impermissible to attend any gathering that consists of that which is prohibited, nor to sit with its people. As for answering an invitation [as in the ḥadīth], it refers to a gathering safe from such prohibited matters. ‘Your brothers’ here refers to the brotherhood of Islām.
If the inviter is an innovator and answering his invitation represents a benefit in terms of inclining him away from his innovation, or being a means of guiding him, then there is nothing wrong with attending his wedding. If attendance does not offer the procurement of such a benefit, then it is more befitting that he does not attend.2
Shaykh ʿAbd al-Muḥsin al-ʿAbbād
[Q]: What is obligatory and recommended with regards to answering wedding invitations?
[A]: Answering wedding invitations is obligatory [in general]. Among the scholars are those who specify only those invited fall under this ruling. As for a person inviting another through someone else saying: Call so-and-so so that he may come to the wedding party, or invites that are issued on cards; are all of these iterations obligatory or are some recommended?
The scholars (رحمهم الله) say: It is obligatory to answer a wedding invitation in the first instance for the one who has been specifically invited whether this invite is issued from the person getting married or through a representative or a card that is sent. This ruling has the prerequisite that the wedding does not consist of that which is objectionable. If the wedding consists of objectionable matters, then this ruling requires some elaboration. If he is able to attend and prevent the occurrence of objectionable matters, then it is obligatory for him to attend. If he is not able to prevent them, then it is impermissible for him to attend.
As for cards that are distributed widely without specification of particular invitees, then it is most apparent to me that it is not obligatory to answer such invitations. This is because many people send such cards to people as a means of extra enjoyment only, or simply to announce the occurrence of a wedding. The exception to this is if one knows that the card was sent and the inviter wishes for him to attend specifically due to some closeness of relation between them, then attendance would be obligatory.
Shaykh Muḥammad ibn Ṣāliḥ al-ʿUthaymīn
 Authentic: narrated by Muslim: 1429
 Source: Sharḥ Sunan Abī Dāwūd 20: 423
Translated by: Riyāḍ al-Kanadī