In the Hadith it is mentioned that Rasulullah (Sallallahu alayhi wasallam) refused to eat milk and honey together.

Answer

This was not because mixing honey with milk is harmful physically or for the health, but because it was too luxurious. Austerity demands that one should not satisfy the desires of the nafs at all times and in all things even if permissible. When Nabi (Sallallahu alayhi wasallam) declined to eat both, he stated explicitly that it was not haraam to have both together. In matters of food, Rasulullah (Sallallahu alayhi wasallam) and his household were very austere.