I said: Messenger of Allah, I am a man who goes out hunting; may I pray in a single shirt? He replied: Yes, but fasten it even if it should be with a thorn.  

قُلْتُ يَا رَسُولَ اللَّهِ إِنِّي رَجُلٌ أَصِيدُ أَفَأُصَلِّي فِي الْقَمِيصِ الْوَاحِدِ قَالَ نَعَمْ وَازْرُرْهُ وَلَوْ بِشَوْكَةٍ  

I witnesses the Messenger of Allah ﷺ praying in a shirt.  

إِنِّي رَأَيْتُ رَسُولَ اللَّهِ ﷺ يُصَلِّي فِي قَمِيصٍ