الْوَقْتُ ثَمِينٌ. فَاحْفَظْهُ.
Time is valuable. Therefore preserve it.
The first clause is a report and the second an imperative. They serve a connected purpose, yet their speech-act functions differ: one can be judged true or false, while the other requests conduct. A shared theme does not mean that wāw should mechanically coordinate all such combinations.
Here fāʾ explicitly indicates a consequence or inference. When considering waṣl by wāw, the issue is whether two separate but appropriately related judgments are intended or whether the directive is being presented as the practical outcome of the assertion.