Sayyiduna Abyad bin Hammal (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him) for the salt mine at Ma'rib. He (peace and blessings be upon him) intended to give it to him, but a man said: "It is like ever-flowing water." So he (peace and blessings be upon him) declined to give it.
Al-Asma'i said: "Al-ma' al-'idd" is water that is perpetual and never ceases, like the water of a spring or a well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب إحياء الموات / 11829