Last night when I was at the mosque, during my first sitting while I was saying (At-tahiyyatu lillahi was-salawatu wat-tayyibat, as-salamu 'alaika ayyuhan-Nabiyyul Arabiy) Allah compliments, prayers and pure words are due to Allah. Peace be upon you, O Arabian Prophet and I bear witness that there is no God but Allah, I did not stop due to inattentiveness and continued to recite the complete prayer of Ibrahim (durud). Because of hastiness I did not realize my mistake but when I stood up for the third rak’at I forgot to recite surah Fatiha bearing in mind that it was a congregational prayer in the mosque. The question is what is the ruling on my prayer? ?أخطأت في التشهد ولم أقرأ الفاتحة فما حكم صلاتي

In the name of Allah, The Most Gracious, The Most Merciful.
Wa alaikum assalam wa rahmatullahi wa barakatuh.
There is no harm in your saying on tashahud: Peace be upon you, O Arabian Prophet because you said it unintentionally so that does not have any effect on your prayer.
As for your forgetting Surah Fatiha in the third rak’ah ; surah Fatiha is a rukn (basic element) of prayer according to a hadith narrated by Bukhari and Muslim on the authority of Ubadah Ibn al-Samit who said: Allah's Messenger (peace be on him) said, “Whoever does not recite Al-Fatiha in his prayer, his prayer is invalid.” As well as the hadith narrated by Muslim on the authority of Abu Hurayra who said: The Messenger of Allah (peace be on him) said, “He who said his prayer, but did not recite the opening chapter of al-Kitab, his prayer is incomplete. He repeated it thrice.” This is the view held by the majority of scholars including the Imams of the schools of thought except Imam Abu Hanifa. So basing on this, it is obligatory that you repeat the prayer now, however if you remember during prayer is compulsory for you to make an extra rak’ah in place of the rak’ah which you did not recite surah Fatiha.
And Allah knows best.

Your brother,
Dr. Khalid al-Muslih
25 / 09 / 1424 AH