Narrated Anas:

When the Prophet (ﷺ) was made to ascend to the Heavens, he said (after his return), "I came upon a river the banks of which were made of tents of hollow pearls. I asked Gabriel. What is this (river?) He replied, 'This is the Kauthar.'

Narrated Abu Ubaida:

I asked `Aisha 'regarding the verse:--'Verily we have granted you the Kauthar.' She replied, "The Kauthar is a river which has been given to your Prophet on the banks of which there are (tents of) hollow pearls and its utensils are as numberless as the stars."

Narrated Abu Bishr:

Sa`id bin Jubair said that Ibn `Abbas said about Al-Kauthar. "That is the good which Allah has bestowed upon His Apostle." I said to Sa`id bin Jubair. "But the people claim that it is a river in Paradise." Sa`id said, "The river in Paradise is part of the good which Allah has bestowed on His Apostle."

Narrated Ibn `Abbas:

The word 'Al-Kauthar' means the abundant good which Allah gave to him (the Prophet (ﷺ) Muhammad). Abu Bishr said: I said to Sa`id, "Some people claim that it (Al-Kauthar) is a river in Paradise." Sa`id replied, "The river which is in Paradise is one item of that good which Allah has bestowed upon him (Muhammad).