Mishkat al-Masabih 3882
Ibn ‘Abbas said:
God’s Messenger was a servant under command who applied to us* only three things which he did not apply to the people in general. He ordered us to perform ablution fully, not to consume sadaqa , and not to make an ass cover a mare. 

* He means the members of the Prophet’s family.

Tirmidhi and Nasa’i transmitted it.
وَعَنِ ابْنِ عَبَّاسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَبْدًا مَأْمُورًا مَا اخْتَصَّنَا دُونَ النَّاسِ بِشَيْءٍ إِلَّا بِثَلَاثٍ: أَمَرَنَا أَنْ نُسْبِغَ الْوُضُوءَ وَأَنْ لَا نَأْكُلَ الصَّدَقَةَ وَأَنْ لَا نَنْزِيَ حمارا على فرس. رَوَاهُ التِّرْمِذِيّ وَالنَّسَائِيّ