When to Say Du`a Al-Istikharah?
Question

When a Muslim prays salat Al-Istikharah, should he recite the Du`a of Istikharah after the Tashahhud and before finishing the prayer, or after saying the Taslim at the end of the prayer?
Praise be to Allah.What is prescribed with regard to Du`a Al-Istikharah is that it should be said after finishing the prayer. This is the view of the majority of scholars. 
It says in Al-Mawsu`ah Al-Fiqhiyyah (3/245): The Hanafis, Malikis, Shafi`is and Hanbalis said: The Du`a should come immediately after the prayer. This is in accordance with what is mentioned in the text of the Hadith from the Messenger of Allah (blessings and peace of Allah be upon him). End quote. 
Shaykh Ibn Baz (may Allah have mercy on him) said: What is prescribed for the Muslim, when he offers Al-Istikharah prayer , is to offer the Du`a after saying the Salam at the end of the prayer, because the Prophet (blessings and peace of Allah be upon him) said: “If any one of you is concerned about a decision he has to make, then let him pray two Rak`ahs of non-obligatory prayer, then say: Allahumma inni astakhiruka bi `ilmika… (O Allah, I seek Your guidance [in making a choice] by virtue of Your knowledge…)”
This indicates that the Du`a should come after saying the Salam at the end of the prayer. (End quote from Majmu` Al-Fatawa, 11/389) 
The scholars of the Standing Committee were asked: Does the Du`a of Istikharah come before the Taslim or after saying the Taslim and finishing the prayer? 
They replied: The Du`a of Istikharah comes after the Taslim of Al-Istikharah prayer. (End quote)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: The Du`a of Istikharah comes after the Salam, because the Prophet (blessings and peace of Allah be upon him) said: “Let him pray two Rak`ahs then let him say…” and he mentioned the Du`a. (End quote from Liqa’ Al-Bab Al-Maftuh, no. 20) 
And Allah knows best.