Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.120B.

There are many authentic hadith which state that it is obligatory to recite al-Fatihah in every rak’ah. Thus, there is no difference of opinion on this point. Some of these hadith are:

‘Ibadah ibn as-Samit related that the Prophet said, “There is no prayer for one who does not recite the opening of the Book al-Fatihah).” This is related by “the group.”

Abu Hurairah reported that the Prophet said, “Whoever prays a prayer and does not recite the opening chapter of the Qur’an has not prayed correctly.” (Related by Ahmad, al-Bukhari and Muslim.) Ad-Daraqutni also recorded a hadith with a sahih chain with almost exactly the same wording.

Said Abu Sa’eed, “We were ordered to recite the opening chapter of the Qur’an and what (else) was easy (for us).” This is related by Abu Dawud. Al-Hafez and Ibn Sayyid an-Nass consider its chain as sahih.

In some of the narrations dealing with the prayer’s incompleteness, it states, “And then recite the ‘Mother of the Book’ (al-Fatihah),” and he said, “And do that in every rak’ah.”

It is confirmed that the Prophet, upon whom be peace, recited al-Fatihah in every rak’ah of every prayer, obligatory or superogatory. Since this is an act of worship, we can only follow what he did. And the Prophet said, “Pray as you have seen me pray.” (Related by al-Bukhari.)