Sending Salutations on the Prophet is an Obligation at least Once in a Lifetime
Question :
Some people consider sending salutations of peace upon the Prophet obligatory at least once, after which, it is recommended.
Answer:
Sending peace and blessings upon the Prophet is an obligation, according to the Command of Allah, the Most Glorified in His Words:
"O you who believe! Send your Salat on (ask Allah to bless) him (Muhammad and (you should) greet (salute) him with the Islamic way of greeting (salutation, i.e. As-Salamu 'Alaikum).
Essentially, it is an obligation, and since there is nothing in the Verse to indicate repetition, the obligation is to do it once in a lifetime. Repetition of it is recommended, based upon the Hadiths which encourage us to do it, except in those cases where the Hadiths prove that it is obligatory.
And may peace and blessings be upon our Prophet Muhammad and upon all his family and Companions.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 1 Pages 276-277