Ayyub عَـلَيْـهِ الـسَّـلَام was commanded to perform Ghusl from that water. When        he عَـلَيْـهِ الـسَّـلَام performed Ghusl, then all diseases of the apparent body disappeared.

Then, he عَـلَيْـهِ الـسَّـلَام walked forty steps. He عَـلَيْـهِ الـسَّـلَام was commanded to strike the ground with his foot again. When he عَـلَيْـهِ الـسَّـلَام did so, then another spring of water emerged whose water was extremely cold. By the command of Allah Almighty, when he عَـلَيْـهِ الـسَّـلَام drank that water, it removed all the inner illnesses of his body and he عَـلَيْـهِ الـسَّـلَام was blessed with a great health. (Khaazin, Taht-al-Ayah, 84, vol. 3, pp. 291)

This bestowment from Allah Almighty upon him was to shower His mercy upon him and to advise worshippers so that through this parable, they become aware of the great reward for observing patience over trials and calamities and they earn reward by observing patience. (Khaazin, Taht-al-Ayah, 84, vol. 3, pp. 291; Madarik, Taht-al-Ayah, 84, pp. 724)

This incident of Sayyiduna Ayyub عَـلَيْـهِ الـسَّـلَام has also been mentioned in the Holy Quran. Allah Almighty states in Part 17, Surah-e-Ambiya, Ayah no. 83 and 84:

وَ اَیُّوْبَ اِذْ نَادٰى رَبَّهٗۤ اَنِّیْ مَسَّنِیَ الضُّرُّ وَ اَنْتَ اَرْحَمُ الرّٰحِمِیْنَۚۖ(۸۳) فَاسْتَجَبْنَا لَهٗ فَكَشَفْنَا مَا بِهٖ مِنْ ضُرٍّ وَّ اٰتَیْنٰهُ اَهْلَهٗ وَ مِثْلَهُمْ مَّعَهُمْ رَحْمَةً مِّنْ عِنْدِنَا وَ ذِكْرٰى لِلْعٰبِدِیْنَ(۸۴)

Translation from Kanz-ul-Iman: And (remember) Ayyoob (Job), when he called his Lord that, ‘Distress has afflicted me, and You are The Most Merciful of all those who do mercy.’ We therefore heard his prayer, so We removed the adversity that had afflicted him, and We gave him his family and in addition, bestowed along with them more of a similar number by Mercy from Ourselves, and advice for the people who worship.

(Part 17, Surah Al-Ambiya, Ayah 83 - 84)