Musar על מכות 47:6

And "[I have returned to Zion, and will dwell in the midst of Jerusalem:] and Jerusalem shall be called the City of Truth" (Zecharia 8:3) - to magnify its importance. And our sages of blessed memory said (Makot 24a): "'who speaks truth in his heart' (Tehilim 15:2), like Rav Safra..." To teach you just how far-reaching the obligation of truth extends.