عَنْ أَبِي عَبْدِ اللٌّهِ (ع) قَالَ: لَوْ عَلِمَ اللٌّهُ شَيْئاً أَدْنَى مِنْ أُفٍّ لَنَهَى عَنْهُ وَ هُوَ مِنْ أَدْنَى الْعُقُوقِ وَ مِنَ الْعُقُوقِ أَنْ يَنْظُرَ الرَّجُلُ إِلَى وَالِدَيْهِ فَيُحِدَّ النَّظَرَ إِلَيْهِمَا.
Imam as-Sadiq (peace be upon him) has said: “Had Allah known of a thing more trivial and insignificant than the word ‘ugh’, he would have forbidden it (to be spoken to the parents). Uttering ‘ugh’ (to the parents) is the mildest form of ill conduct towards the parents. One of the ways in which a person can be regarded as being disowned by the parents is that he casts a hard and stern look upon them.”
Al-Kafi, Volume 4, Page 50