Assalamualaikum ustaz. What is the ruling for a person who recites al-Fatihah without following the tajwid in prayer? Is his prayer valid? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Basically, in madhhab al-Syafie, reciting al-Fatihah in prayer is one of the requisites of prayer and it is obligatory to be recited in every rakaat, regardless of whether it is the obligatory or sunnah prayer. If a person doesn’t recite al-Fatihah in any rakaat of his prayer, then his prayer is invalid. This is clear through the statement of the Prophet PBUH:
لَا صَلَاةَ لِمَنْ لَمْ يَقْرَأْ بِفَاتِحَةِ الْكِتَابِ
“He who does not recite Fatihah al-Kitab is not credited with having observed prayer.” 
Likewise, in a hadith narrated from Abu Hurairah RA, Rasullullah PBUH said:
مَنْ صَلَّى صَلاَةً لَمْ يَقْرَأْ فِيهَا بِفَاتِحَةِ الْكِتَابِ فَهْىَ خِدَاجٌ. يَقُولُهَا ثَلاَثًا
“He who said his prayer, but did not recite the opening chapter of al-Kitab, his prayer is incomplete. He repeated it thrice.” 
Imam al-Nawawi said: Reciting al-Fatihah is obligatory in all prayers whether the obligatory or sunnah prayers. If a person forgets to recite al-Fatihah, the obligation isn’t abrogated. This is agreed upon and has been stated by jumhur scholars who are from madhhab Syafie. 
Coming back to the above question, surah al-Fatihah must be recited correctly following the right recitation being mindful of the tajwid and makhraj. Mistakes in the tajwid such as the recitation of mad (long or short recitation), ghunnah (nasal sound) or other tajwid mistakes don’t invalidate prayer, however, the recitation should be improved over time by learning from a person of calibre and eligible.
This is as stated by Allah SWT:
وَرَتِّلِ ٱلۡقُرۡءَانَ تَرۡتِيلًا
“And recite the Qur’an with measured recitation.” 
Al-Syeikh al-Tantawi in his commentary al-Wasit stated that what is meant by ‘وَرَتِّلِ ٱلۡقُرۡءَانَ تَرۡتِيلًا’ is to recite it in a euphonious voice, humbly submissive (khusyuk) and with reflection (tadabbur) as well as have a determination to recite it perfectly, being mindful of the makhraj of the letters, waqaf, mad, izhar, ikhfa’ and other rulings pertaining it.
As for the recitation that invalidates a prayer is if the recitation is wrong and it changes the meaning. If a person recites أَنْعَمْتُ (an’amtu) with a dhommah (diacritic sign – recited with a short vowel [U] sound) purposely, when the correct recitation is ‘أَنْعَمْتَ’ (an’amta) with a fathah (diacritic sign with a short [A]) sound, then his prayer is invalid. If he forgot, then he should repeat his recitation.
Mistakes in the recitation related to the tajwid rulings are known as Lahn al-Khafiy (hidden mistakes) while Lahn al-Jaliy (clear mistakes) is mistakes that change the meaning, letter and diacritic sign of the verse, these types of mistakes can invalid prayer.
According to the hadith ‘’, it can be understood that ‘’ means good recitation. Thus, let us all learn the Quran whether in terms of its recitation or to practice its contents.
 Narrated by al-Bukhari (756) dan Muslim (900)
 Narrated by Muslim (907)
 See al-Majmu’ Syarh al-Muhazzab, 3/288
 Surah al-Muzammil: 4