Rinsing the mouth thoroughly is from the perfection of wudu except when one is fasting

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."  

قُلْتُ يَا رَسُولَ اللَّهِ أَخْبِرْنِي عَنِ الْوُضُوءِ قَالَ أَسْبِغِ الْوُضُوءَ وَخَلِّلْ بَيْنَ الأَصَابِعِ وَبَالِغْ فِي الاِسْتِنْشَاقِ إِلاَّ أَنْ تَكُونَ صَائِمًا