Wa'Laikum Assalaam Yoga is a spiritual practice of Hindus. There is a saying that goes something like this "There is no Yoga without Hinduism, and there is no Hinduism without Yoga". Yoga is an integral part of their religion and so you may want to speak with a Sheikh about it's permissibility. Also I don't even know how you could learn yoga in a Halal environment (unless it were at home) due to the kind of dress that Yoga calls for, and public classes at gyms or athletic centres are obviously open to women of all different sexual orientations. You can't go wrong when you ask a Sheikh.