This isn’t an easy one because there are risks to both decisions here.
The most important question if you do feel the need to tell is why? Are you looking for validation from your partner for your ability not to cave in to temptation? Do you want them to be proud of you because you didn’t take an opportunity to cheat? Do you want to make a statement to them that you are super considerate? If so challenge if you should be seeking a reward for not doing something you shouldn’t have been doing in the first place? Confessing this temptation that you have declined might be being honest with your partner but could also bring up other issues of insecurity depending on where their mindset is or frustration if you are seeking an validation of your refusal.
The other issues is if you don’t tell them and they find out. Maybe it was someone you know who made the offer. Maybe you told a friend. Will they feel betrayed if someone else tells them their partner was offered to cheat? Your refusal might not be celebrated rather the fact that you kept something from them seeing you be punished. It might also deliver the idea that there was more to the story than just an offer and maybe something to hide. Often relationships are based on honesty and your decision to tell or not should be based on the type and amount of honesty you and your partner have.
If you do want to tell, sometimes the casual funny approach is best. It takes away any seriousness from the situation and if you do see it as a funny situation it will show there was not even a moment of consideration on your behalf.