Nothing is perfect, but it is bound to deteriorate

Ibn Jarir recorded that Harun bin `Antarah said that his father said, "When the Ayah,

﴾الْيَوْمَ أَكْمَلْتُ لَكُمْ دِينَكُمْ﴿

'This day, I have perfected your religion for you...' was revealed, during the great day of Hajj (the Day of Arafah, the ninth day of Dhul-Hijjah) Umar cried. The Prophet said, "What makes you cry?" He said, "What made me cry is that our religion is being perfected for us. Now it is perfect, nothing is perfect, but it is bound to deteriorate." The Prophet said,

«صَدَقْت»

"You have said the truth.'' 

What supports the meaning of this Hadith is the authentic Hadith:

«إِنَّ الْإِسْلَامَ بَدَأَ غَرِيبًا، وَسَيَعُودُ غَرِيبًا، فَطُوبَى لِلْغُرَبَاء»

Islam was strange in its beginning and will return strange once more. Therefore, Tooba [good tidings] for the strangers. [Sahih Muslim]

Source: Tafsir Ibn Kathir