´It was narrated that Anas bin Malik said:` "The Messenger of Allah milked a sheep and drank some of its milk, then he called for water and rinsed his mouth and said: 'It has some greasiness in it.'"
Hadith Referenceسنن ابن ماجه / كتاب الطهارة وسننها / 501
Hadith Gradingالألبانی:ضعيف | زبیر علی زئی:ضعيف, إسناده ضعيف, زمعة بن صالح: ضعيف وضعفه الجمهور كما قال البوصيري, انوار الصحيفه، صفحه نمبر 395
Narrated Anas ibn Malik: The Messenger of Allah ﷺ drank some milk and he did not rinse his mouth nor did he perform ablution, and he offered the prayer.
Shaykh Umar Farooq Saeedi
Benefits and Issues: It is recommended (mustahabb) and superior to rinse the mouth after drinking milk; however, if one does not do so, it is still permissible.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 197