Narrated As-Sa'b bin Jath-thama: An onager was presented to Allah's Messenger (ﷺ) at the place called Al-Abwa' or Waddan, but Allah's Apostle rejected it. When the Prophet (ﷺ) noticed the signs of sorrow on the giver's face he said, "We have not rejected your gift, but we are in the state of Ihram." (i.e. if we were not in a state of Ihram we would have accepted your gift, Fath-ul-Bari page 130, Vol. 6)
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، عَنِ الصَّعْبِ بْنِ جَثَّامَةَ ـ رضى الله عنهم ـ أَنَّهُ أَهْدَى لِرَسُولِ اللَّهِ صلى الله عليه وسلم حِمَارًا وَحْشِيًّا وَهْوَ بِالأَبْوَاءِ أَوْ بِوَدَّانَ فَرَدَّ عَلَيْهِ، فَلَمَّا رَأَى مَا فِي وَجْهِهِ قَالَ " أَمَا إِنَّا لَمْ نَرُدَّهُ عَلَيْكَ إِلاَّ أَنَّا حُرُمٌ ".
sunnah.com