What rule number mentions preflight?  

Good point. In addition, "Preflight" is all over the FARs, just a quick word search will verify that. Certainly it's required and described in the RFM. Lastly, here is a cut-n-paste from the AIM of which the number one cause is interesting: 

7–6–1 Accident Cause Factors

a. The 10 most frequent cause factors for general aviation accidents that involve the pilot-in-command are:

1. Inadequate preflight preparation and/or planning.

2. Failure to obtain and/or maintain flying speed.

3. Failure to maintain direction control.

4. Improper level off.

5. Failure to see and avoid objects or obstructions.

6. Mismanagement of fuel.

7. Improper inflight decisions or planning.

8. Misjudgment of distance and speed.

9. Selection of unsuitable terrain.

10. Improper operation of flight controls.