Sayyiduna Abdullah bin al-Zubayr says: Sending blessings (salat) upon the Prophet (peace and blessings be upon him) on the minbar on Friday is not part of the Sunnah.
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Layth bin Abi Sulaym, who is a mudallis (one who conceals defects in his chain).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 3149