Little black flies on house plant?

[Confuzzeled]

There's little black flies on one of my house plants, I put it outside for a few days but they're still there.

Does anyone know how to get rid of them?

[Lizzzombie]

I was told to spray a mixture of washing up liquid & water onto them, as it shouldn't kill the plant but will get rid of the pests. (it worked on my tomatoes)

[Confuzzeled]

fab. thanks.