‘Abdul Muttalib himself says:

I was once asleep in the Hateem area when a person came to me in a dream and instructed, “Dig up Barrah (that which is full of blessing).” As I enquired, “What is Barrah?” he departed. On the second day I was sleeping in the same spot when the same man again directed me in my dream, “Go and dig up Al-Madhnunah (that which is treasured).” When I asked him, “What is Al-Madhnunah?” he went away. On the third day, I was sleeping in the same spot when he again appeared in my dream and commanded, “Go and dig up Tayyibah (that which is pure).” As I enquired about Tayyibah, he once again left. On the fourth day, he ordered me, “Go and dig up Zam Zam.” Again I asked, “What is Zam Zam?” To this he replied, “It is a well, the water of which neither runs out nor decreases in volume, and it provides countless pilgrims with drinking water.” He then went on to point out a few distinctive clues precisely indicating where I should dig.”

The recurring nature of the dream coupled with a detailed location of the area convinced ‘Abdul Muttalib that this was a true dream.

(Extracted from Seeratul Mustafa 1/35)