Musnad Ahmad 32:1481

'Abd al-Wahhab ibn 'Ata' Abu Nasr al-'Ijli al-Khaffaf narrated to us, he said: Sa'id informed us, from Qatadah, from Anas ibn Malik, that he informed them, from the Prophet, may Allah bless him and grant him peace, that he said: "While I was walking in Paradise, a river appeared before me, its banks were domes of hollowed pearl." He said: So I said: "O Gabriel, what is this?" He said: "This is the Kawthar which your Lord has given you." He said: "So I struck my hand into it, and its mud was the pungent musk, and its pebbles were pearls." And 'Abd al-Wahhab said from his book, I read: "The angel who was with me said: 'Do you know what this is? This is the Kawthar which your Lord has given you.' So he struck his hand into its ground and brought out from its mud the musk."