I saw the Messenger of Allah ﷺ praying (Nafl prayer) on a donkey's back while his face was turned towards Khaibar. (Using translation from Muslim 700e)  

«رَأَيْتُ النَّبِيَّ ﷺ يُصَلِّي عَلَى حِمَارٍ وَهُوَ مُتَوَجِّهٌ إِلَى خَيْبَرَ»  

"The Messenger of Allah ﷺ sent me on an errand then I came back to him while he was praying. I greeted him with the salam and he gestured to me. When he finished he called me and said: 'You greeted me with Salam just now and I was praying.' And he was facing toward the east that day." (Using translation from Nasāʾī 1189)   

بَعَثَنِي رَسُولُ اللَّهِ ﷺ فِي حَاجَةٍ فَأَدْرَكْتُهُ فَسَلَّمْتُ عَلَيْهِ وَهُوَ يُصَلِّي فَأَشَارَ إِلَيَّ فَلَمَّا فَرَغَ دَعَانِي فَقَالَ «إِنَّكَ سَلَّمْتَ عَلَيَّ وَأَنَا أُصَلِّي» وَهُوَ مُتَوَجِّهٌ يَوْمَئِذٍ نَحْوَ الْمَشْرِقِ  

[Machine] He heard Ibn Umar say, "The Messenger of Allah ﷺ used to pray on his mount wherever it turned in the journey."  

أَنَّهُ سَمِعَ ابْنَ عُمَرَ يَقُولُ «كَانَ رَسُولُ اللَّهِ ﷺ يُصَلِّي عَلَى رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ فِي السَّفَرِ»  

[Machine] He said, "We were with the Messenger of Allah ﷺ on a journey, and he sent me on a mission. So, I came to him while he was walking, and I greeted him, and he nodded with his hand. Then, I greeted him again, and he gestured but did not speak to me. Then, he called me later and said, 'I was performing a voluntary prayer.'"  

أَنَّهُ قَالَ كُنَّا مَعَ رَسُولِ اللَّهِ ﷺ فِي سَفَرٍ فَبَعَثَنِي مَبْعَثًا فَأَتَيْتُهُ وَهُوَ يَسِيرُ فَسَلَّمْتُ عَلَيْهِ فَأَوْمَأَ بِيَدِهِ ثُمَّ سَلَّمْتُ فَأَشَارَ وَلَمْ يُكَلِّمْنِي فَنَادَانِي بَعْدُ وَقَالَ «إِنِّي كُنْتُ أُصَلِّي نَافِلَةً»  

"The Messenger of Allah ﷺ sent me on an errand, then I came back to him while he was facing the east or the west. I greeted him with salam and he gestured to me. Then when he finished he called me and said: "O Jabir!" So I came and said: "O Messenger of Allah, I greeted you with Salam but you did not answer." He said: 'I was praying.' (Using translation from Nasāʾī 1190)   

بَعَثَنِي رَسُولُ اللَّهِ ﷺ مَبْعَثًا فَوَجَدْتُهُ يَسِيرُ مُشْرِقًا وَمُغْرِبًا فَسَلَّمْتُ عَلَيْهِ فَأَشَارَ بِيَدِهِ ثُمَّ سَلَّمْتُ عَلَيْهِ فَأَشَارَ بِيَدِهِ فَانْصَرَفْتُ فَنَادَانِي «يَا جَابِرُ» فَنَادَانِي النَّاسُ يَا جَابِرُ فَأَتَيْتُهُ فَقُلْتُ يَا رَسُولَ اللَّهِ قَدْ سَلَّمْتُ عَلَيْكَ فَلَمْ تَرُدَّ عَلَيَّ قَالَ «ذَاكَ أَنِّي كُنْتُ أُصَلِّي»  

[Machine] "I saw the Messenger of Allah ﷺ praying on a riding animal towards the east in the Battle of Anmar."  

«رَأَيْتُ رَسُولَ اللَّهِ ﷺ يُصَلِّي عَلَى رَاحِلَةٍ نَحْوَ الْمَشْرِقِ فِي غَزْوَةِ أَنْمَارٍ»  

[Machine] "We were with the Messenger of Allah ﷺ in a battle, and he used to pray voluntary prayers on his mount facing the east. When he wanted to pray the obligatory prayers, he would dismount and face the Qibla."  

«كُنَّا مَعَ رَسُولِ اللَّهِ ﷺ فِي غَزْوَةٍ فَكَانَ يُصَلِّي تَطَوُّعًا عَلَى رَاحِلَتِهِ مُسْتَقْبِلَ الْمَشْرِقِ فَإِذَا أَرَادَ أَنْ يُصَلِّيَ الْمَكْتُوبَةَ نَزَلَ وَاسْتَقْبَلَ الْقِبْلَةَ»  

[Machine] I saw the Prophet ﷺ praying on his riding animal during travel, making a movement with his head as he was on a pulpit.  

«رَأَيْتُ النَّبِيَّ ﷺ يُصَلِّي عَلَى دَابَّتِهِ فِي السَّفَرِ فِي السُّبْحَةِ يُومِئُ بِرَأْسِهِ إِيمَاءً»  

[Machine] He heard Jabir saying, "I saw the Prophet ﷺ praying on his mount and he was praying the supererogatory prayers on every side, but he would lower his head in prostration for the two prostrations of each unit of prayer by nodding."  

أَنَّهُ سَمِعَ جَابِرًا يَقُولُ «رَأَيْتُ النَّبِيَّ ﷺ وَهُوَ يُصَلِّي عَلَى رَاحِلَتِهِ يُصَلِّي النَّوَافِلَ فِي كُلِّ وَجْهٍ وَلَكِنَّهُ يَخْفِضُ السَّجْدَتَيْنِ مِنَ الرَّكْعَتَيْنِ يُومِئُ إِيمَاءً»  

[Machine] "I saw the Prophet ﷺ praying while riding his camel, performing voluntary prayers in every direction. However, he would slightly lower his prostration during the two prostrations of each unit, gesturing with a nod."  

«رَأَيْتُ النَّبِيَّ ﷺ يُصَلِّي وَهُوَ عَلَى رَاحِلَتِهِ النَّوَافِلَ فِي كُلِّ وَجْهٍ وَلَكِنَّهُ يَخْفِضُ السَّجْدَتَيْنِ مِنَ الرَّكْعَةِ يُومِئُ إِيمَاءً»  

[Machine] "I saw the Prophet ﷺ praying the voluntary prayers on his mount, lowering his head during the two prostrations of the two units of prayer."  

«رَأَيْتُ النَّبِيَّ ﷺ يُصَلِّي النَّوَافِلَ عَلَى رَاحِلَتِهِ يَخْفِضُ السَّجْدَتَيْنِ مِنَ الرَّكْعَتَيْنِ»