Hazrat Ibn Abbas (Radiyallahu 'anh) reports that the Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) arrived in Madinah and found the Jews observing fast on the day of ‘Ashura… They said: “It is the day of great (significance) when Allah delivered Hazrat Musa ('Alaihis-Salaam) and his people and drowned Pharoah and his people, and Sayyidina Musa ('Alaihis-Salaam) observed fast out of gratitude. And we also observe it.” The Holy Messenger of Allah (Sall Allahu alaihi wa Aalihi wa Sallim) responded: “We have more right, and we have closer connection with Sayyidina Musa ('Alaihis-Salaam) than you have”; so Allah’s Messenger (Sall Allahu alaihi wa Aalihi wa Sallim) observed fast (on the day of ‘Ashura) and gave us orders to observe it. [Sahih Bukhari and Muslim]
Re: Fasting on the 10th of muharram
if he s.a.w gave us the order to fast it must be farz like Ramazan ul mubarik ? or is it mustahib like fasts of shaban ?