As per the hadith of prophet SAW:
Book of Revelation
Bukhari :: Book 1 :: Volume 1 :: Hadith 1
Narrated ‘Umar bin Al-Khattab:
I heard Allah’s Apostle saying, “The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended. So whoever emigrated for worldly benefits or for a woman to marry, his emigration was for what he emigrated for.”
Also reported in Sahih Muslim
So in the light of this hadith your intention i.e. your act of thinking about offering prayer in heart, is actually your niyat, and you are not required to say that niyat words.
And Allah knows the best.