Question 201: Some people believe that it is obligatory to invoke peace and blessings upon the Prophet (peace be upon him) once in their lifetime and anything that exceeds this is Mustahab (desirable).
Answer: Invoking Allaah’s peace and blessings upon the Prophet is Fard (obligatory, based on a definitive text), for Allaah’s Command in His Saying (Exalted be He): O you who believe! Send your Salaah on (ask Allaah to bless) him (Muhammad صلى الله عليه وسلم), and (you should) greet (salute) him with the Islaamic way of greeting (salutation i.e. Al-Salaamu ‘Alaykum ). The basic principle concerning the imperative mood is that it denotes obligation or duty. Since the command in the Aayah does not denote repetition, obligation (to do this) must be once in a lifetime and recurrence is desirable, based on the Hadeeths mentioned in this regard indicating the desirability of doing so (i.e., invoking peace and blessings upon the Prophet), except at the places specified by Hadeeths as being obligatory therein... read more here.