Abudhar Ghiffari was with the expedition, but the animal he rode could not stand the blazing sun of the desert.  He trailed behind his animal plodding forward at a slow pace, till at last he had to abandon the animal and walk.

        Someone said: "O Prophet, Abu Dhar has deserted us."
        The Holy Prophet (SAW) answered: "Allah's Will be done.  If Abu Dhar wishes to be with us he will soon appear."
 
        Abu Dhar set forth to reach the Holy Prophet (SAW) but the scorching heat of the desert worked against him.  As he persisted, he came to an oasis.  He tasted the water and found it cold and sweet.

        "You cannot quench your thirst while the Holy Prophet (SAW) might be thirsty," he said to himself, and filled his water skin.  At last, he drew near and the companions not recognizing him from far informed the Holy Prophet (SAW) that someone was approaching.

        "Perhaps it is Abu Dhar," the Holy Prophet said.
        Abu Dhar collapsed near the feet of the Holy Prophet (SAW) totally spent and tired by the heat and the thirst.  The Holy Prophet (SAW) ordered:
"Give him some water".

        Abu Dhar said: "I have water with me".
        "Then why did you not drink?"  The Holy Prophet (SAW) demanded.
        "Because I found it sweet and cold; and I thought that the Messenger of Allah should drink it first, before I do."

Reference:
Pearls of Wisdom.  A String of Incidents in the History of Islam.  The World Federation of Khoja Shia Ithna-Asheri Muslim Communities. Page 18.