It is permissible to prostrate on blankets etc.
Question

                My grand mother says its not permissible to do sajda on a soft surface like on a blanket while praying i.e one should do sajdah on a hard surface.Is it true?.
Praise be to Allah.
    It is permissible to prostrate on soft surfaces like blankets 
    etc., so long as the worshipper can place his forehead and nose firmly on 
    the surface when prostrating. What is not allowed is prostrating on 
    something too soft, in which the worshipper’s face will sink and not be 
    firmly placed.  

    Ibn al-Hamaam said in Fath al-Qadeer (1/304): It is 
    permissible to prostrate on grass, straw and cotton, if it is firmly laid on 
    the ground, and likewise compacted snow, but not if his face will sink into 
    it or it is not firmly laid on the ground. End quote. 

    Ibn al-Qayyim said in al-Bahr al-Raa’iq (1/337): The 
    basic principle is that just as it is permitted to prostrate on the ground, 
    it is also permitted to prostrate on anything that is like the ground, where 
    his forehead may be firmly placed, meaning that if he presses hard, his head 
    will not sink any further down. End quote. 

    Based on this, there is nothing wrong with prostrating on an 
    item like a blanket or mattress, if it is firm and will remain stable no 
    matter how much pressure a person puts on it. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was 
    asked about the ruling on prostrating on a sponge pad. He replied: If the 
    sponge is thin and will flatten out when he prostrates, then there is 
    nothing wrong with it. 

    Fataawa Ibn ‘Uthaymeen 
    (13/184). 

    And Allaah knows best.