Question: Can I pray sunnah/nafila sitting down without a reason?
Yes, but one receives only half the reward on one standing. He may also enter sitting and stand thereafter. Or begin standing and then sit when he wants. However, if he intends to pray it all standing then he must complete it standing.
Written by Dr. Shadee Elmasry
Source: Matn al-Akhdari
وَأَمَّا النَّافِلَةُ فَيَجُوزُ لِلْقَادِرِ عَلَى الْقِيَامِ أَنْ يُصَلِّيَهَا جَالِسًا، وَلَهُ نِصْفُ أَجْرِ الْقَائِمِ. وَيَجُوزُ أَنْ يَدْخُلَهَا جَالِسًا وَيَقُومَ بَعْدَ ذَلِكَ أَوْ يَدْخُلَهَا قَائِمًا وَيَجْلِسَ بَعْدَ ذَلِكَ إِلَّا أَنْ يَدْخُلَهَا بِنِيَّةِ الْقِيَامِ فِيهَا فَيَمْتَنِعَ جُلُوسُهُ بَعْدَ ذَلِكَ.
