The Essence
Al-Quddus means the One who is absolutely pure, flawless, and free from any imperfection or deficiency. He is far above any human weakness, making Him worthy of ultimate glorification and worship.
In the Quran
يُسَبِّحُ لِلَّهِ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ الْمَلِكِ الْقُدُّوسِ الْعَزِيزِ الْحَكِيمِ
"Whatever is in the heavens and whatever is on the earth is exalting Allah, the Sovereign, the Pure (Al-Quddus), the Exalted in Might, the Wise. (Surah Al-Jumu'ah 62:1)"
In Your Duas
سُبُّوحٌ قُدُّوسٌ رَبُّ الْمَلاَئِكَةِ وَالرُّوحِ
"Glorious, Holy (Quddus), Lord of the angels and the Spirit."
— Sahih Muslim 487