Has Adi Shankaracharya ever said something about mantra japa?

This is not about Mantra Japa, but Adi Shankara has explained in detail about Nama japa in his commentary on Sri Vishnu Sahasranama (the book with Adi Shankara's original Sanskrit commentary and its hindi translation is available from Gita Press, Gorakhpur). I am quoting few verses from the same,. read more

Yes he said about Om Namah Shivaya mantra japa and commnicated to his disciples.The guru parampara of Amnaya peetas still follow this method of Communication to their disciples. The authentication for this is a book written by Padmapadacharya a prime disciple of Sri Adishankaracharya. read more

Bhargo the light, devasya of the deity, dheemahi we should meditate. The rishi instructs us to meditate upon the available form (light of suns) to discover the formless Creator (God). Also he wants us to do japa of the word Om (this is understood in the Mantra). read more

Choosing a mantra. Choose a mantra you have an affinity for, use it regularly, and stick with it, since changing mantras diminishes their effect. It is also more effective to use an existing mantra that has a history and tradition, rather than making something up. The simplest Sanskrit mantra is Om, or Aum. read more