Tesla was included in the International System of Units by the decision of the XI General Conference on Weights and Measures in 1960, simultaneously with the adoption of SI as a whole.

1960
Tesla - unit for magnetic flux density, intensity and magnetic field induction in the International System of Units (SI).
In accordance with the rules of SI, and in relation to the derived units are named after scientists, the name of the unit "Tesla" is written in lower case, and its designation in capital letters (TL). It's a bit different in practice.