Shiva Purana
यच्चंदनैश्चंदनकेपि मिश्रं धार्यं हि भस्मैव त्रिपुंड्रभस्मना । विभूतिभालोपरि किंचनापि धार्यं सदा नो यदि संतिबुद्धयः 1-24-५० ।
Just as sandal paste alone can be applied over sandal paste, so also only the ash shall be applied over the sacred mark on the forehead. A sensible person will not apply anything over the forehead that wears the ornamental mark of ashes on it.
english translation
yaccaMdanaizcaMdanakepi mizraM dhAryaM hi bhasmaiva tripuMDrabhasmanA । vibhUtibhAlopari kiMcanApi dhAryaM sadA no yadi saMtibuddhayaH 1-24-50 ।
hk transliteration by Sanscript