Assalamualaikum ustaz. I missed both my Zohor and Asar prayers when I was travelling previously. Should I replace my prayers in full (each with four rakaat) or just qadha with two rakaat prayers each?
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.
Islam permits a traveller who is on a journey of more than two marhalah to qasar or shortens the rakaat for prayers which is permissible for qasar such as Zohor, Asar and Isyak. Islam also permits a traveller to gather two obligatory prayers at one time which is known as jamak prayers. There are two categories of jamak which are; jamak taqdim and jamak ta’khir.
Qasar prayer is a rukhsah (facilitation) given during travels. This is based on a question asked by Umar bin al-Khattab to Rasullullah PBUH. Ya’la bin Umayyah said to Umar RA:
لَيْسَ عَلَيْكُمْ جُنَاحٌ أَنْ تَقْصُرُوا مِنَ الصَّلاَةِ إِنْ خِفْتُمْ أَنْ يَفْتِنَكُمُ الَّذِينَ كَفَرُوا فَقَدْ أَمِنَ النَّاسُ فَقَالَ عَجِبْتُ مِمَّا عَجِبْتَ مِنْهُ فَسَأَلْتُ رَسُولَ اللَّهِ -صلى الله عليه وسلم- عَنْ ذَلِكَ. فَقَالَ صَدَقَةٌ تَصَدَّقَ اللَّهُ بِهَا عَلَيْكُمْ فَاقْبَلُوا صَدَقَتَهُ
“You may shorten the prayer,” only “if you fear those who are infidels may afflict you” (Al-Qur’an; 4:101), whereas the people were now safe. He replied that he had wondered about the same matter, so he asked God’s Messenger and received the reply, “It is an act of charity which God has done to you, so accept His charity.” 
Syeikh ‘Abd al-Muhsin al-‘Abbad said: “Hence, it is permissible for a traveller to shorten the four rakaat prayers to two rakaat and jamak the prayers of zohor and asar, and maghrib and isyak. This is the charity given to all of us.” 
Syeikh Dr Wahbah al-Zuhaili when commenting on the wisdoms of the rukhsah of qasar and jamak is to ease the burdens of travel and facilitate a person to fulfil the obligation. 
In answering the above question, we begin with the statement of the Prophet PBUH:
إِذَا رَقَدَ أَحَدُكُمْ عَنِ الصَّلاَةِ أَوْ غَفَلَ عَنْهَا فَلْيُصَلِّهَا إِذَا ذَكَرَهَا فَإِنَّ اللَّهَ يَقُولُ أَقِمِ الصَّلاَةَ لِذِكْرِي
“When any one of you omits the prayer due to sleep or he forgets it, he should observe it when he remembers it, for Allah has said:” Observe prayer for remembrance of Me.” 
According to the scholars of madhhab Hanafi and Maliki, a person has to qadha his missed prayers according to the time the prayer is missed. Thus, a traveller has to qadha the prayer he missed when he was a resident in full (four rakaat), while a resident has to qadha the prayer he missed when he was travelling by qasar (two rakaat). 
Whilst, according to madhhab Syafie, the azhar qaul is for the prayer missed during qasar to be qadha in qasar if he is travelling and in full if he is residing. However, there are three other opinions on this issue:
- Qadha in full regardless of whether he is travelling or residing.
- Qadha by qasar regardless of whether he is travelling or residing.
- Qadha by qasar if he is travelling and in full is he is residing or on another journey. 
According to madhhab Hanbali, it has to be qadha in full, for it is a rukhsah in travels and the rukhsah is automatically abrogated once he arrived. Whereas, Imam Ahmad issued a fatwa to perform the prayer in full as an ihtiyat (precautionary step) and this is also the opinion of Imam al-Auza’ie and Daud al-Zahiri.
To conclude, we are inclined towards the opinion of madhhab Syafi’e and Hanbali, it is permissible to qadha the missed prayer during travels by qasar on the condition that he is still travelling. As for if he is now residing somewhere, then he should qadha in full. Likewise, if he missed his prayers when he was residing and wanted to replace the prayer when he is travelling, then he should do so in full.