مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 348
Purification
How to act when relieving oneself - Section 2
وَعَن عَائِشَة قَالَتْ: كَانَتْ يَدُ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ الْيُمْنَى لِطَهُورِهِ وَطَعَامِهِ وَكَانَتْ يَدُهُ الْيُسْرَى لِخَلَائِهِ وَمَا كَانَ مِنْ أَذًى. رَوَاهُ أَبُو دَاوُد
‘A'isha said, “God’s messenger used his right hand for his ablution water and his food, and his left hand for his evacuations and anything objectionable.” Abu Dawud transmitted it.
Mishkat al-Masabih, 348صَحِيح (الألباني)