I have been trying to look for it but they are all bad quality and have too may add or are not English.
We have a list of Legal Streaming Sources here. considering that Aniplex licensed Magi for the states their streaming channel has Season 1 and Season 2, assume that the Magi you have tagged is the same as the one you mention in the title. (as Season 1 is called Magi: The Labyrinth of Magic and Season is is called Magi: The Kingdom of Magic)
I am unable to confirm whether or not these episodes are dubbed in English as Geo-blocking prevents non US users from viewing the videos (i am in Australia), however considering DVD Set 1 has English listed as a spoken language and it's publishing is listed as Aniplex of America i would suspect that the episodes on their streaming channel are English Dub
EDIT: Thanks to senshin's comment i have come to realize that the series and Aniplex of America has aren't what you are looking for. looking on Wikipedia and Anime News Network neither lists any english distributors/companies/licencees for the either the 2014 OVA or the current Adventure of Sinbad anime adaptation of the manga of the same name. this leads me to suspect that it is not officially released outside Japan at this time (17/04/2016) so i would suspect a dub for these does not exist.
That being said generally when a company like Funimation or Aniplex gets the licenses to stream/distribute a series there is a high chance that they will also get any new season (though there are some exceptions). so at this time it's probably best to keep an eye on Aniplex of America for if/when they post up the dubs for the series your looking for. checking the other sites in the list i posted wont hurt either just incase