'A’isha reported that when any person fell ill with a disease or he had any ailment or he had any injury, the Apostle of Allah (may peace be upon him) placed his forefinger upon the ground and then lifted it by reciting the name of Allah. (and said): The dust of our ground with the saliva of any one of us would serve as a means whereby our illness would be cured with the sanction of Allab. This hadith has been transmitted on the authority of Ibn Abu Shaiba and Zubair with a slight variation of wording. [Muslim]
Sahl b. Sa’d reported that Allah’s Messenger (may peace be upon him) said on the Day of Khaibar: I would certainly give this standard to a person at whose hand Allah would grant victory and who loves Allah and His Messenger and Allah and His Messenger love him also. The people spent the night thinking as to whom it would be given. When it was morning the people hastened to Allah’s Messenger (may peace be upon him) all of them hoping that that would be given to him. He (the Holy Prophet) said: Where is 'Ali b. Abu Talib? They said: Allah’s Messenger, his eyes are sore. He then sent for him and he was brought and Allah’s Messenger (may peace be upon him) applied saliva to his eyes and invoked blessings and he was all right, as if he had no ailment at all, and coraferred upon him the standard. 'Ali said: Allah’s Messenger, I will fight them until they are like us. Thereupon he (the Holy Prophet) said: Advance cautiously until you reach their open places, thereafter invite them to Islam and inform them what is obligatory for them from the rights of Allah, for, by Allah, if Allah guides aright even one person through you that is better for you than to possess the most valuable of the camels. [Muslim]
Narrated Abu Said Al-Khudri:
Some of the companions of the Prophet came across a tribe amongst the tribes of the Arabs, and that tribe did not entertain them. While they were in that state, the chief of that tribe was bitten by a snake (or stung by a scorpion). They said, (to the companions of the Prophet ), "Have you got any medicine with you or anybody who can treat with Ruqya?" The Prophet's companions said, "You refuse to entertain us, so we will not treat (your chief) unless you pay us for it." So they agreed to pay them a flock of sheep. **One of them (the Prophet's companions) started reciting Surat-al-Fatiha and gathering his saliva and spitting it (at the snake-bite). The patient got cured and his people presented the sheep to them, but they said, "We will not take it unless we ask the Prophet (whether it is lawful)." When they asked him, he smiled and said, "How do you know that Surat-al-Fatiha is a Ruqya? Take it (flock of sheep) and assign a share for me."**
[Bukhari]
Again, it’s like taweez, we don’t put trust in the taweez to help us, we put trust in Allah. Just like medication, we take it but doesn’t mean THAT will cure us, Allah does. So same thing with dumm wala pani we take it because Allah’s words are blown on it. Allah knows best.