Your Question: I know some Muslims (mostly) Shias put a stone on their Masalla where the head falls. I think they call it Turbat or Khaq-a-sifa - what is this

The literal meaning of the Arabic term ‘Turbah or Turbat’ is derived from the root word ‘Turb’ meaning ‘dust’. 

The term ‘Khaaq’ is not even an Arabic term, but rather an Urdu term which again means ‘dust’; and ‘Shifa’ means ‘something that heals’;  thus ‘Khaaq-e-Shifa’ means the ‘dust that heals’!!!

It is indeed true that some sects amongst the Muslim Ummah place this ‘dust’ or ‘stone’ on their prayer mat where their head lands in the state of prostration. 

The best of what we have heard from the people who follow this practice is that this ‘Turbah’ or ‘Khaaq’ is the dust or stone from the lands of Makkah, Madinah, or Karbala in Iraq; for they believe that the dust of no land is worth resting one’s head while in prostration other than the above three places!

Your Question: can you help me proof to them that it is a Bidaah. 

Dear and Beloved Brother, logic dictates that proof and evidence can only be presented when one wishes for the Truth to be confirmed; there can be no proof or evidence presented for a thing based on falsehood! 

Besides, it is the ones who practice the ‘bida’ or innovation who should be asked to present the evidence or proof for their act,  if indeed they believe that they are following the truth!

If one claims to believe in Allah and His Last and Final Messenger (saws), then it is incumbent upon them to follow the Quran and the Sunnah in all their acts of the deen.  If one follows an act which is not endorsed by the Quran and the Sunnah of the Messenger of Allah (saws), then it is obvious that it is an innovation or a ‘bida’ of man and nothing to do with the Truth.  Allah is our witness, there is absolutely nothing in the Holy Quran or in the Sunnah of the Messenger of Allah (saws) whereby he (saws) himself prostrated on a ‘turbah’ or ‘khaaq-e-shifa’ or he (saws) ever advised his noble companions or his family to do the same! 

The onus of presenting the proof and evidence should be on the one who practices something in the deen which is not endorsed by Allah and His Messenger (saws); and not on the true believers who follow the Truth of the Quran and the Sunnah.

Prostration on a ‘turbah’ or ‘Khaaq-e-shifa’ is without an iota of a doubt an innovation which was brought into the deen long after the death of Prophet Mohamed (saws); and cannot be considered a part of the Truth of Islam. 

Sahih Muslim Hadith 1885     Narrated by Jabir ibn Abdullah

That the Prophet of Allah (saws) said in a sermon:  “The best speech is that which is embodied in the Book of Allah, the Al Quran;  and the best guidance is the guidance given by me, Mohamed (saws). The most evil affairs are the innovations, and every innovation is an error.”

Sunan of Abu-Dawood Hadith 4515  Narrated by Ali ibn AbuTalib

That the Prophet of Allah (saws) said: “If anyone introduces an innovation (in religion),  he will be responsible for it.  If anyone introduces an innovation or gives shelter to a man who introduces an innovation (in religion), he is cursed by Allah,  by His Angels,  and by all the people.”