Muhammad bin al-Munkdar bin 'Abdullah محمد بن المنكدر بن عبد الله بن الهدير · The Book on Al-Witr

"Allah's Messenger would teach us Al-Isthikhara for all of our affairs just as he would teach us a Surah of the Qur'an, saying: 'When one of you is worried about a matter, then let him perform two Rak'ah other than the obligatory (prayer), then let him say: (Allahumma inni astakhiruka bi'ilmika, wa astaqdiruka biqudratika, wa as'aluka min falikal-azim, fa innaka taqdiru wa la qadiru, wa ta'lami wa la a'lamu, wa anta allamul-ghayub. Allahumma in kunta ta'lamu anna hadhal-amra khairun li…