It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) came to the Siqayah (the place where the pilgrims are given water to drink) and asked for water. Abbas (may Allah be pleased with him) said to Fadl (may Allah be pleased with him): "Go to your mother and bring a drink from her house for the Messenger of Allah (peace and blessings be upon him)." He (peace and blessings be upon him) said: "Give me to drink from here." It was said: "O Messenger of Allah! People put their hands in it." He (peace and blessings be upon him) said: "Give me to drink." So he (peace and blessings be upon him) drank from it. Then he (peace and blessings be upon him) came to Zamzam, where the people were drawing water and working, and he said: "Keep working, for you are engaged in a righteous deed." Then he said: "Were it not for the fear that you would be overwhelmed by the crowd, I would come down myself and place this on my shoulder," meaning that he pointed towards his shoulder.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11913