Musnad Ahmad 32:1461

Yunus and Hassan ibn Musa narrated to us, they said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Anas ibn Malik: That some people mentioned the Hawd (the Cistern) in the presence of Ubayd Allah ibn Ziyad—Hassan said: Ali ibn Zayd, from Al-Hasan, that the Hawd was mentioned in the presence of Ubayd Allah ibn Ziyad—so he denied it and said: "What is the Hawd?" This reached Anas ibn Malik, so he said: "Indeed, by Allah, I will certainly do something." He went to him and said: "Did you mention the Hawd?" Ubayd Allah said: "Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention it?" He said: "Yes, more than such and such a number of times. He says: 'Indeed, the distance between its two sides is as the distance between Ayla and Makkah—or between San'a and Makkah—and its vessels are more numerous than the stars of the sky.'" Hassan said: "And its vessels are more numerous than the number of the stars of the sky."