The admoniion in last part of Hadith 18 of an-Nawawi is

Hadith 18 of an-Nawawi is the famous Hadith of Gabriel, in which Angel Gabriel visited the Prophet Muhammad (peace be upon him) and asked him about the pillars of Islam, the pillars of faith, and the signs of the end of times. The admonition at the end of this Hadith is about good conduct or Ihsan, which means excellence in worship and in daily life. The Prophet Muhammad (peace be upon him) said, "Ihsan is to worship Allah as if you see Him, and if you do not see Him, then He sees you." This means that one should always strive to do their best in worship and in their actions, as if they are always in the presence of Allah, and that Allah is always watching them. It is a reminder to maintain good conduct and to do everything in the best possible way, with sincerity and with the intention of pleasing Allah.