Many shia people in mumbai keep fast (roza)1st to 9th moharram? I want to know why they keep and is it sawab to keep and which Imaam told?

1. Fasting on 1st day of Muharram highly recommended. Imam Ridha (pbuh) asks his companion, Ar Rayyan ibn Shabeeb – “Are you fasting?” He answers no. Imam then says: “Yabna Shabeeb! This is the day when Zakariyya at 75 years prayed to his Rabb (3:38) for a child. Allah accepts his dua and commands angel to go to his mihraab and announce the good news of a son Yahya (3:39). One who fasts and invokes Allah He will accept as the dua of Zakariyya”
2. It is mustahab to fast on the first nine days of the month.