Rinsing the mouth thoroughly is from the perfection of wudu except when one is fasting
TIRMIDHI 788:
from ʿĀṣim b. Laqīṭ b. Ṣabrah from his father —
From his father who said: "I said. 'O Messenger of Allah! Inform me about Wudu.' So he said: "Perform Wudu well, and go between the fingers, and perform Istinshaq extensively except when fasting."
الترمذي ٧٨٨:
عن عاصم بن لقيط بن صبره عن أبيه —
قُلْتُ يَا رَسُولَ اللَّهِ أَخْبِرْنِي عَنِ الْوُضُوءِ قَالَ أَسْبِغِ الْوُضُوءَ وَخَلِّلْ بَيْنَ الأَصَابِعِ وَبَالِغْ فِي الاِسْتِنْشَاقِ إِلاَّ أَنْ تَكُونَ صَائِمًا