Likely the sump gasket. They seem to crack and slide out of place on these behind the left head.

Changed the oil and filter?

If yes, could be you pinched the oil filter gasket.

+1 on the sump gasket. I have 2006 24hp Intek with 400 hrs and just had to replace the sump gasket due to very small leak. With very careful inspection you may be able to see oil weeping. While engine is cold, remove plastic shroud and clean area where you suspect the leak. I also removed the metal heat shield. Run engine briefly and inspect. Shut engine down and inspect again. My leak was noticable after shutting down engine. Very small leak but enough to cause a mess. My leak was to the left of the oil filter near the cylinder on the same side as filter. I originally suspected the oil filter had a bad seal but that was not the case. Be careful when separating sump case and keep an eye on parts within - particularly governor. I am now having an issue with high reving - possibly governor related, since my repair. Good luck.
Paul