I'm looking for a general bound (in terms of $p$) for the Kloosterman sum, working in $\mathbb{F}_{p}$,
$$\sum\limits_{x_{1} \dots \ x_{n} = a} \psi(x_{1} + \dots + x_{n})$$ for $\psi$ a nontrivial additive character.
These are well studied, and I've found papers that discuss them which give bounds, but unfortunately I can't seem to find a single agreed upon answer and indeed in what I've found there appears to be contradictory information.
Does someone know what the "correct" bound is, and/or know of a good source?