Answered by Shaykh Faraz Rabbani
salam alaikum,
what must one do in a fard/nafl prayer when he is reciting a surah after the fatiha and he accidently mixes a few lines of a dua (in arabic) with the recitation. must he perform sajda sahw?
jazakAllah khayr for your answer

Walaikum assalam wa rahmatullah,
I pray that this finds you well, and in the best of health and spirits. May Allah grant you all good and success in this life and the next.
Though improper in obligatory prayers, making supplication between one’s recitation of Qur’an doesn’t affect the validity or soundness of one’s prayer.
And Allah alone gives success.
Faraz Rabbani