An Overlooked Sunnah

In the Name of Allaah, Ar-Rahmaan, Ar-Raheem...

‎عن ابنِ عَبَّاسٍ : أنَّ النَّبِيَّ (صلى الله عليه و سلم ) شَرِبَ لَبَناً فَدَعاَ بِمَاءٍ فَتَمَضْمَضَ ثُمَّ قَالَ : (( إِنَّ لَهُ دَسَمًا )).

Ibn 'Abbaas (may Allah be pleased with him) narrated the Prophet (peace and blessings be upon him) drank some milk then called for some water. Once he got the water he rinsed his mouth then said, "It has fat."  [Collected by Bukharee 211 and Muslim 358]

Benefits for this subject:
Imaam An-Nawwawee said, "This hadeeth confirms it is recommended to rinse the mouth after drinking milk." [Sharh Saheeh Muslim]

Abu Daawood mentioned this hadeeth in his Sunnan. He wrote the chapter- Wudhoo' from Milk. However he didn't intend the technical meaning for wudhoo', but rather he meant the linguistic meaning for wudhoo'. [Bathul-Majhud fee halee Sunnan Abee Daawood]

Shaykh al-'Uthaymeen said, "The reason why the Prophet (peace and blessings be upon him) rinsed his mouth was because of the fat. It can be understood from this action that it's legislated to rinse the mouth from all foods which contain fat, regardless whether drank or chewed. It is good manners to rinse the mouth afterwards to remove the fat. If the substance is heavy in fat, then it's better to use siwak. For this reason the scholars say:  it is sunnah to use the siwak to remove any fat after eating if the smell of the mouth changed. [ Sharh Saheeh Al-Bukahree]