Milk
Quick Answer
Innate purity (Fitrah), lawful wealth, and beneficial knowledge.
Classical Interpretation (Ibn Sirin)
Drinking sweet, fresh milk in a dream is one of the most praiseworthy symbols. It represents the Fitrah (innate, pure nature of Islam), sound knowledge, and Halal (lawful) wealth. Drinking sour or spoiled milk signifies unlawful money or corruption in one's religion. Spilling milk indicates a loss of knowledge or wealth.
Hadith Context
"In the dream of the Night Journey (Isra and Mi'raj), the Prophet (PBUH) was offered a cup of wine and a cup of milk. He chose the milk, and Jibril said, 'You have chosen the Fitrah.' (Sahih Bukhari)"