Prophets of Allah are protected from committing blasphemy before and after Prophethood. The saying of Ibrahim about the planet when he saw it:﴿هذا ربي﴾  is a form of expression in Arabic called negative interrogative (استفهام انكاري)  i.e. a way of expressing one’s strong denial of a matter by posing a question which clearly shows that the matter is ridiculous or totally unacceptable. It is as if he said: “Is this my Lord, as you claim? [Undoubtedly, it is not].” Prophet Ibrahim already knows that Lordship cannot be attributed to other than Allah. Allah ta^ala said in Surat Al ^Imran verse #67:

﴿ما كان إبراهيم يهودياً ولا نصرانياً ولكن كان حنيفاً مسلماً وما كان من المشركين﴾ سورة ءال عمران.

The verse means: [Abraham was neither a Jew nor a Christian, but a Muslim on the straight path and did not associate a partner with Allah].