Question: A man wanted to purchase some goods from a merchant, so the latter gave him three different models to choose from. The man asked him to tell honestly which was the best, so the merchant said, “Honestly, this is the best one.” Neither of them intended by this to swear by honesty, they merely wanted to entrust each other with the duty of telling each other the truth. Is this considered Kufr (disbelief) and atheism?
Answer: If none of them intended by the word “honestly” swearing by other than Allaah, but just entrusting his Muslim brother with telling him the truth, there is nothing wrong with this at all. However, they should not have used this expression which in a literal sense means swearing by honesty… read more here.