As a viewer suggested there can be found comments that this fragment belonged to St. Polyeuktos. On this site can be found a picture of a fragment very similar to the one on my picture. The caption says: "Note: Found during construction work in Istanbul, this marble block with an inscription provided the first clue to the identification of the church of St. Polyeuktos. The fragmentary inscription was identified as part of the dedicatory inscription of the church which was recorded and preserved in the so-called Anthologia Palatina, a collection of inscriptions from buildings and other monuments in the city of Constantinople. St. Polyeuktos was built around 524-7 by Anicia Juliana and stood adjacent to her own palace. "
And on the same page is a picture of one of the columns in front of the Baptistery of St. Marc in Venice, which I know pretty well and have slides of.
For Venice fans the article I found about another part of the church in la serenissima .