Musnad Ahmad 5:571

Isma'il ibn Ibrahim narrated to us, Ayyub informed us, from Mujahid, who said: Ali said: I was once hungry in Medina with a severe hunger, so I went out seeking work in the high grounds of Medina. I found a woman who had gathered clay, and I thought she wanted to moisten it. I came to her and contracted with her for every bucket [of water] for a date. I drew sixteen buckets until my hands became blistered. Then I went to the water and drank from it. Then I came to her and said with my palm like this before her—and Isma'il spread his hands and brought them together—"so she counted out for me sixteen dates." I came to the Prophet, peace and blessings of Allah be upon him, and informed him, and he ate with me from them.