Wa Alaykum Assalam and thank you for your question.

The worldly reward is good health if you follow a balanced diet at the same time. Milk contains protein for your muscles and calcium for your bones as well as many other health benefits that are only found in milk; especially fresh milk, not pasteurized.

Concerning reward in the hereafter, we read in the hadith that the Prophet (peace be upon him) liked drinking milk; therefore, if you do so intending to follow the Prophet’s sunnah then you will be rewarded accordingly.

However, there is no mention in any authentic Islamic sources what is the reward for drinking milk. There is, however, for offering milk to others:

He who gives a she-camel as a gift, for him is the reward (of the gift) both morning and evening – a reward for drinking milk in the morning and a reward for drinking milk in the evening. (Sahih Muslim)

“Whoever gives someone some milk or silver, or guides him through a strait, then he will have the reward similar to freeing a slave.” (At-Tirmidhi – Sahih)

I hope this helps answer your question.

Salam and please keep in touch.