Allah (Exalted be He) says in the Glorious Qur'an: (Part No. 6; Page No. 419)
So, recite you of the Qur’ân as much as may be easy for you. He knows that there will be some among you sick
Does the recitation mentioned in this Ayah relate to the Nafilah (supererogatory) or the obligatory Salah (Prayer)?
The Ayah which reads:
Verily, your Lord knows that you do stand (to pray at night) a little less than two thirds of the night, or half the night, or a third of the night, and also a party of those with you. And Allâh measures the night and the day. He knows that you are unable to pray the whole night, so He has turned to you (in mercy). So, recite you of the Qur’ân as much as may be easy for you.
Was revealed regarding the night Salah. It is reported that the Prophet (peace be upon him) said to the one who did not offer his Salah well:
When you stand for prayer, say Takbir (saying: "Allahu Akbar [Allah is the Greatest]") and then recite from the Qur’an as much as may be easy for you...
The Prophet (peace be upon him) commanded him to recite as much as may be easy for him after saying Takbirah Al-Ihram (Saying Allahu Akbar at the beginning of the Salah) i.e. to recite Al-Fatihah. This is an obligation and Salah is a general word which includes the obligatory and Nafilah Salah. So, this is a proof of the obligation of reciting Al-Fatihah in Salah. The Hadith of `Ubadah ibn Al-Samit explained and confirmed this as he narrated that the Prophet (peace be upon him) said:
The prayer of whoever does not recite Surah Al-Fatihah is invalid
Muslim, and others.
(Part No. 6; Page No. 420) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.