Well, I was studying previous papers where I found that members are quite confused regarding first mosque built and the year when Zakat was made obligatory. However, when Prophet (PBUH) was on the way towards Madina he stayed at Quba where he offered prayer and built a mosque namely "Masjid-e-Quba" and Masjid-e-Nabawi was built in 1st Hijri. As the question says After the Hijrat to Madinah the 1st mosque constructed by Prophet PBUH was Masjid-e-Nabawi as it isn't asking that while travelling towards Madinah Prophet PBUH built 1st Mosque. Owing to that, Zakat was made obligatory in 2nd Hijri so the correct one is 2nd Hijri.

I know it's too late to reply, but its for the new aspirants so that they have the clear outlook.

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~~ Khan ~~
Birds born in cage, thinks flying is an illness.