What is the reference of consider bhagwatpad Adi Shankaracharya to be an incarnation of shiva ? Did he proclaimed himself to be so anywhere ?
Well, many Puranas mention Adi Shankara to be an incarnation of Shiva.
Lord Shiva in the Padma Purana, Uttara Khanda, Chapter 236 states:
mayavadam asac chastram pracchannam bauddham uchyate mayaiva kalpitam devi kalau brahmana rupina (Padma Purana 6.236.7)
Translation: "Mayavada or Advaita philosophy is an impious, wicked belief and against all the conclusions of the Vedas. It is only covered Buddhism. My dear Parvati, in Kali-Yoga I assume the form of a brahmana (Adi Shankara) and teach this imagined philosophy.
In the 30th chapter of the Kurma Purana too, Adi Shankaracharya is mentioned an incarnation of Shiva:
तदेव साधयेन्नृणां देवतानां च दैवतम्। करिष्यत्यवताराणि शंकरो नीललोहितः॥३३॥
श्रीतस्मार्तप्रतिष्ठार्थ भक्तानां हितकाम्यया। उपदेक्ष्यंति तज्ज्ञानं शिष्याणां ब्रह्मसंज्ञितम्॥ ३४॥
सर्ववेदान्तसारं हि धर्मान्वेदनिदर्शितान्। सर्ववर्णान् समुद्दिश्य स्वधर्मा ये निदर्शिताः।।३५।।
Translation: The people should adore Lord Mahadeva, who happens to be the god of gods. The brown complexioned Shiva, for the establishment of Srauta (Vedic) and Smarta dharmas (belonging to the Smritis), and for the welfare of the devotees shall incarnate on earth. He will enlighten his devotees on the subjects of the gist of Vedantas, the knowledge relating to the Brahman, and all the dharmas enshrined in the Vedas, which have been narrated earlier.