Mishkat al-Masabih 373
Marwan al-Asfar said:
I saw Ibn ‘Umar make his camel kneel facing the qibla , then sit down and pass water in its direction, so I said,
“Abu ‘Abd ar-Rahman, [Ibn ‘Umar's kunya ] has this not been forbidden?” He replied, “No, that was forbidden only in open country; but when there is something between you and the qibla to conceal you, there is no harm.” 

Abu Dawud transmitted it.
عَن مَرْوَان الْأَصْفَر قَالَ: «رَأَيْتُ ابْنَ عُمَرَ أَنَاخَ رَاحِلَتَهُ مُسْتَقْبِلَ الْقِبْلَةِ ثُمَّ جَلَسَ يَبُولُ إِلَيْهَا فَقُلْتُ يَا أَبَا عَبْدِ الرَّحْمَنِ أَلَيْسَ قَدْ نُهِيَ عَنْ هَذَا قَالَ بلَى إِنَّمَا نُهِيَ عَنْ ذَلِكَ فِي الْفَضَاءِ فَإِذَا كَانَ بَيْنَكَ وَبَيْنَ الْقِبْلَةِ شَيْءٌ يَسْتُرُكَ فَلَا بَأْس» . رَوَاهُ أَبُو دَاوُد