From Sahih Bukhari (Hadith 2573):
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)
Reference: Bukhari 2573