Hadith no. 209 is a repeat. Read it here.
Narrated Ibn ‘Abbas:
Allah’s Apostle drank milk, rinsed his mouth and said, “It has fat.”
We all know that, right? But there’s another point to it. Prophet (SAW) is giving the reason for rinsing his mouth. It has fat. Get it?
So, the sunnah to be derived is: rinse your mouth before praying if you’ve eaten a fat-containing food. :)