`Uqbah bin `Amir (May Allah be pleased with him) said: One day the Messenger of Allah (PBUH) went out and asked Allah's forgiveness for the martyrs of the battle of Uhud after eight years. It seemed that by so doing, he bid farewell to the living and the dead. He then came back, rose to the pulpit and said, "I shall be your precursor; I am a witness for you (before Allah), and I will be present before you at the River (Haud Al-Kauthar). By Allah I can see with my own eyes the Haud from this place. I am not afraid that you will associate anything with Allah in worship after (my demise), but I apprehend that you will vie with one another for the life of the world.'' The narrator said: It was the last time that I saw the Messenger of Allah (PBUH).
[Al-Bukhari and Muslim].

There are some more narrations with very minor changes in its wording.

Commentary: This Hadith highlights the following points:
1. Haud Al-Kauthar exists: Anas (May Allah be pleased with him) reported: When the Prophet (PBUH) was made to ascend to the heavens, he said (after his return), "I came upon a river (in Jannah) the banks of which were made of tents of hollow pearls. I asked Jibril (PBUH), `What is this (river)?' He replied, `This is the Kauthar.''' (Al-Bukhari, vol. 6, Hadith No. 488).
2. Warning against vying with one another for the world.
3. Permissibility of visiting the graves of the Muslims and praying for them.