Hadith 11 of an-Nawawi admonishes Muslims to avoid doubtful things. The hadith teaches that halal (permissible) and haram (prohibited) things are clear, but there are some things in between that are doubtful or uncertain. Therefore, Muslims should stay away from these doubtful things to ensure that they do not fall into haram. The hadith emphasizes the importance of being cautious and mindful of one's actions, and not taking unnecessary risks. In summary, the hadith encourages Muslims to adhere to a clear and straightforward path and avoid doubtful things that could potentially lead to sin or harm.