قَالَ جَابرُ بْنُ عَبْدِ اللَّهِ رَضِيَ اللَّهُ عَنْهُمَا: كَانَ رسُولُ اللَّهِ صلى الله عليه وسلم يُعَلِّمُنَا الْاسْتِخَارَةَ فِي الْأُمُورِ كُلِّهَا كَمَا يُعَلِّمُنَا السُّورَةَ مِنَ الْقُرْآنِ, يَقُولُ: (إِذَا هَمَّ أَحَدُكُمْ بِالْأَمْرِ فَلْيَرْكَعْ رَكْعَتَيْنِ مِنْ غَيْرِ الْفَرِيضَةِ, ثُمَّ لْيَقُلْ: اللَّهُمَّ إِنِّي أَسْتَخِيرُكَ بِعِلْمِكَ, وَأَسْتَقْدِرُكَ بِقُدْرَتِكَ, وَأَسْأَلُكَ مِنْ فَضْلِكَ العَظِيمِ؛ فَإِنَّكَ تَقْدِرُ وَلاَ أَقْدِرُ, وَتَعْلَمُ وَلاَ أَعْلَمُ, وَأَنْتَ عَلاَّمُ الغُيُوبِ, اللَّهُمَّ إِنْ كُنْتَ تَعْلَمُ أَنَّ هَذَا الأمْرَ - وَيُسَمِّي حَاجَتَهُ - خَيْرٌ لِي فِي دِينِي وَمَعَاشِي وَعَاقِبَةِ أَمْرِي – أَوْ قَالَ: عَاجِلِهِ وَآجِلِهِ - فَاقْدُرْهُ لِي وَيَسِّرْهُ لِي ثمَّ بَارِكْ لِي فِيهِ, وَإِنْ كُنْتَ تَعْلَمُ أَنَّ هَذَا الْأَمْرَ شَرٌّ لِي فِي دِينِي وَمَعَاشِي وَعَاقِبَةِ أَمْرِي – أَوْ قَالَ: عَاجِلِهِ وَآجِلِهِ – فَاصْرِفْهُ عَنِّي وَاصْرِفْنِي عَنْهُ وَاقْدُرْ لِيَ الْخَيْرَ حَيْثُ كَانَ, ثُمَّ أَرْضِنِي بِهِ) وَمَا نَدِمَ مَنِ اسْتَخَارَ الْخَالِقَ, وَشَاوَرَ الْمَخْلُوقِينَ الْمُؤْمِنِينَ وَتَثَبَّتَ فِي أَمْرِهِ, فَقَدْ قَالَ اللَّهُ سبحانه وتعالى: ﴿وَشاوِرْهُمْ فِي الْأَمْرِ فَإِذَا عَزَمْتَ فَتَوَكَّلْ عَلَى اللَّهِ﴾
Jabir bin 'Abdullah (may Allah be pleased with them both) said: The Messenger of Allah (peace be upon him) used to teach us to seek the best choice (Istikharah) in all matters, just as he would teach us a chapter of the Qur'an. He said: 'When any one of you is concerned about a decision he intends to make, let him pray two rak'ahs other than the obligatory prayer, then say: O Allah, I seek Your guidance by Your knowledge, and I seek strength by Your power, and I ask You of Your immense bounty; for verily You have power and I have none, and You know and I know not, and You are the Knower of the unseen. O Allah, if You know this matter — (and here he names his need) — to be good for me in my religion, my livelihood, and the outcome of my affairs — (or he said: in this life and the Hereafter) — then decree it for me, make it easy for me, and then bless it for me. And if You know this matter to be bad for me in my religion, my livelihood, and the outcome of my affairs — (or he said: in this life and the Hereafter) — then turn it away from me and turn me away from it, and decree for me what is good wherever it may be, and then make me content with it.' And he should mention his need. No one who seeks guidance from the Creator, takes counsel with the believers among His creation, and is firm in his affair, will regret it, for Allah, the Exalted, has said: 'and consult them in the affair; then, when you have made a decision, put your trust in Allah.'
Source: Al-Bukhari 7/162, Al-'Imran 3:159.