Bismillāhirraḥmānirraḥīm should be recited at the start of every rak’at (unit of prayer). Many people do not know this ruling that it is sunnah to recite bismillāh[irraḥmānirraḥīm] at the start of every rak’at, i.e. just like in the first rak’at after thanā, a’ūdhubillāh[iminashayṭānirrajīm] and then bismillāh[irraḥmānirraḥīm] is recited. Likewise, even in the second rak’at before reciting sūrah fātiḥa, it is sunnah to recite bismillāh[irraḥmānirraḥīm].

(Bayān at-Tafsīr, pg 56, By Mufti Mahmood Ashraf Usmani (May Allah envelope him with His infinite mercy))

Rendered into English by Mufti Sufyan