Al-Kāfi - Volume 6 > Excellence of the Water of Zamzam and the Water of al-Mizab
Hadith #2

وَ بِإِسْنَادِهِ قَالَ ذُكِرَتْ زَمْزَمُ عِنْدَ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) فَقَالَ أُجْرِيَ إِلَيْهَا عَيْنٌ مِنْ تَحْتِ الْحِجْرِ فَإِذَا غَلَبَ مَاءُ الْعَيْنِ عَذُبَ مَاءُ زَمْزَمَ .

2. Through the same chain of narrators as that of the previous Hadith, the following is narrated: “Once, Zamzam was mentioned before abu ’Abd Allah, ‘Alayhi al-Salam. He (the Imam) said, ’A fountain from under the stone was made to flow on it. When the water of the fountain overwhelms it, water of Zamzam becomes sweet.’”