Allah, ta^ala said in the Qur’an:

وَقَضَى رَبُّكَ أَلاَّ تَعْبُدُواْ إِلاَّ إِيَّاهُ وَبِالْوَالِدَيْنِ إِحْسَانًا إِمَّا يَبْلُغَنَّ عِندَكَ الْكِبَرَ أَحَدُهُمَا أَوْ كِلاَهُمَا فَلاَ تَقُل لَّهُمَآ أُفٍّ وَلاَ تَنْهَرْهُمَا وَقُل لَّهُمَا قَوْلاً كَرِيمًا

Wa qada Rabbuka alla ta^budu illa iyyahu wa bil-walidayni ihsana. Imma yablughanna ^indakal-kibara ahaduhuma aw kilahuma fala taqul lahuma uffiw wala tanharhuma waqul lahuma qawlan karima.

Which means: {Allah orders not to worship except Him and to treat one’s parents kindly. If one of them is alive and old or both are alive (and old), do not say “uff” to them and do not address them with harsh words. Be humble and compassionate and address them with nice and gentle words.}

Allah, ta^ala, made it unlawful to say to one's parents “uff ”, which indicates one is bothered by them. Allah ordered one to say to one’s parents a generous and gentle saying, more so when they are old. Also, Allah orders to speak to one’s parents in a respectful manner.