عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: «مَنْ حَدَّثَكُمْ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَبُولُ قَائِمًا فَلَا تُصَدِّقُوهُ مَا كَانَ يَبُول إِلَّا قَاعِدا» . رَوَاهُ أَحْمد وَالتِّرْمِذِيّ وَالنَّسَائِيّ

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.