Allaah ﷻ said,
{إِنَّ ٱللَّهَ وَمَلَـٰٓئِكَتَهُۥ يُصَلُّونَ عَلَى ٱلنَّبِىِّ ۚ يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ صَلُّوا۟ عَلَيْهِ وَسَلِّمُوا۟ تَسْلِيمًا}
“Indeed, Allaah confers blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [Allaah to confer] blessing upon him and ask [Allaah to grant him] peace.” (Al-Ahzaab 56)
This is thus an order, and the principle for orders is that it is an obligation. However, does that constitute repetition in the sense that it is obligatory to send salutations upon the Prophet ﷺ whenever the reason arises and he is mentioned? Alternatively, is the obligation and sin removed after sending the salutation once, and the salutation for more than one mention is preferred any time he is mentioned more than once? This is an issue of difference of opinion among scholars. Undoubtedly, the principle behind commands is obligation, given that there is a report which is a proof for this, from the words of the Prophet ﷺ,
“The miser is the person to whom when I am mentioned in his gathering, they do not send salutations upon me.” (Ahmad1736),
and the words of the Prophet ﷺ,
“If I am mentioned in the gathering of a person and they do not send salutations upon me, May Allaah bring their nose down into the dust.”
(Tirmithy, 3545, and Ahmad, 7451)