Jami` at-Tirmidhi 89
Ibn Abbas narrated :
the Prophet drank milk. Then he called for water to rinse out his mouth. Then he said: "Indeed it has fat." [He said:] There are narrations on this topic from Sahl bin Sa'd As-Sa'idi and Umm Salamah. 
Abu Eisa said: [And] this Hadith is Hasan Sahih.
Some of the people of knowledge held the view that one is to wash the mouth after drinking milk and this is recommended according us. Some others did not hold the view that one is to wash out the mouth after drinking milk.
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنْ عُقَيْلٍ، عَنِ الزُّهْرِيِّ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم شَرِبَ لَبَنًا فَدَعَا بِمَاءٍ فَمَضْمَضَ وَقَالَ ‏
"‏ إِنَّ لَهُ دَسَمًا ‏"‏ ‏.‏ قَالَ وَفِي الْبَابِ عَنْ سَهْلِ بْنِ سَعْدٍ السَّاعِدِيِّ وَأُمِّ سَلَمَةَ ‏.‏ قَالَ أَبُو عِيسَى وَهَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏ وَقَدْ رَأَى بَعْضُ أَهْلِ الْعِلْمِ الْمَضْمَضَةَ مِنَ اللَّبَنِ وَهَذَا عِنْدَنَا عَلَى الاِسْتِحْبَابِ وَلَمْ يَرَ بَعْضُهُمُ الْمَضْمَضَةَ مِنَ اللَّبَنِ ‏.‏