In Purcell's Electricity and magnetism, page 137, the author derived a formula describing the average velocity $\overline{\textbf{u}} $ of positive ions inside a conductor, a gas made of neutral molecules as well as the ions, when subject to an electric field $\textbf{E}$, and it is given by
$$\overline{\textbf{u}}= \frac{e\textbf{E}\overline{t}}{M}$$
where $M$ is the mass of the ion, $e$ the elementary charge, and $\overline{t}$ is the mean time before a collision occurs between an ion an a neutral molecule within the gas.
He then went on to say, about this formula,
This shows that the average velocity of a charge carrier is proportional to the force applied to it. If we observe only the average velocity, it looks as if the medium were resisting the motion with a force proportional to the velocity. That is the kind of frictional drag you feel if you try to stir thick syrup with a spoon, a "viscous" drag. Whenever charge carriers behave like this, we can expect something like Ohm's law.
How did the author infer from the formula the existence of a frictional force? The relation, as far as I can see, only says the resulting average velocity is proportional to the applied field, it doesn't inform us about the existence or non-existence of a frictional force that opposes this very field.