Yes, it's true! On the authority of 'Abdullaah ibn Mas'ood (may Allaah be pleased with him), the Prophet (sallallaahu 'alayhe wa sallam) said:

"Drink cow's milk! For verily it (the cow) eats from all kinds of vegetation, and it (the milk) is a cure for every illness." [1]

[1] It was collected by Al-Haakim (#8274). Al-Albaanee called it Saheeh in Saheeh Jaami' As-Sagheer (#4059).

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