Deriving from a terminal word in a context free grammar Just to make it clear.
(since my book doesn't mention anything like this)

Suppose we have a context free grammar $G=(V,T,P,S)$.
where $T=\{a,b\}$ (The other sets doesn't really matter).

Since $\Rightarrow^*$ is the reflexive transitive closure of the relation $\Rightarrow$ on the set $(V\cup T)^*$, I see no problem in writing a thing like $ab \Rightarrow^* ab$ (where $ab\in T^*$ and there are no variables on the left hand side of the arrow) since the $\Rightarrow^*$ relation is reflexive.

Now I want to ask wether it common to write a thing like $ab \Rightarrow^* ab$?

By definition I see no problem.

Thanks.
There is absolutely no problem writing $ab \Rightarrow^* ab$. It definitely happens in proofs.