The proposal was included in Norway's 2018-2029 National Transport Plan, which was first made public earlier this spring (Find PDFs here). Of course, unlike the Tesla CEO's Tweet, though (embedded below), Norway has not yet agreed to ban gas cars. The country is trying to increase the number of zero-emission vehicles by 2025 by limiting the number of new fossil-fuel vehicle sales and increasing alternative options, like bike super highways. As CNBC notes, only about 150,000 new vehicles are sold in Norway each year, which means that it's not that difficult for the majority of new vehicles can be electrified in some way, as they were in March.
Whether or not Norway does ban gas cars in the next decade remains to seen, but there's no doubt that if any country is going to take this stand, Norway has a good chance to be the first. The EV scene is strong in Norway thanks to a slew of government incentives like reduced taxes and the ability to drive in bus-only lanes. There is also a strong contingent of EV advocates there, which has influenced politicians.
Just heard that Norway will ban new sales of fuel cars in 2025. What an amazingly awesome country. You guys rock!! pic.twitter.com/uAXuBkDYuR— Elon Musk (@elonmusk) June 3, 2016