Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.032.
It is a sunnah that, if one must choose between permissible alternatives, one may pray two non-obligatory rak’at, even if they are of the regular sunnah prayers or a prayer for entering the mosque, and so on, during any time of the day or night, and to recite therein whatever one wishes of the Qur’an after reciting al-Fatihah. Then one praises Allah and sends salutations to the Prophet sallallahu alehi wasallam and recites the following supplication which has been recorded by al-Bukhari in Jabir’s narration: “The Prophet sallallahu alehi wasallam would teach us al-istikhara for all of our affairs as he would teach us a surah from the Qur’an. He said: ‘If one of you is deliberating over an act, he should pray two non-obligatory rak’at and say:
“O Allah, I consult You as You are All-Knowing and I appeal to You to give me power as You are Omnipotent, I ask You for Your great favour, for You have power and I do not, and You know all of the hidden matters . O Allah ! If you know that this matter (then he should mention it) is good for me in my religion, my livelihood, and for my life in the Hereafter, (or he said: ‘for my present and future life,’) then make it (easy) for me. And if you know that this matter is not good for me in my religion, my livelihood and my life in the Hereafter, (or he said: ‘for my present and future life,’) then keep it away from me and take me away from it and choose what is good for me wherever it is and please me with it.”
There is nothing authentic concerning something specific that is to be recited in the prayer nor is there any authentic report concerning how many times one should repeat it.
An-Nawawi holds that “after performing the istikharah, a person must do what he is wholeheartedly inclined to do and feels good about doing and should not insist on doing what he had desired to do before making the istikharah. And if his feelings change, he should leave what he had intended to do, otherwise he is not completely leaving the choice to Allah, and would not be honest in seeking aid from Allah’s power and knowledge. Sincerity in seeking Allah’s choice, means that one should completely leave what he himself had desired or determined.”