Muwatta Imam Malik Book of Business Transactions, Hadith 60
Yahya related to me from Malik from Nafi that Abdullah ibn Umar bought a female riding-camel for four camels and he guaranteed to give them in full to the buyer at ar-Rabadha.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، اشْتَرَى رَاحِلَةً بِأَرْبَعَةِ أَبْعِرَةٍ مَضْمُونَةٍ عَلَيْهِ يُوفِيهَا صَاحِبَهَا بِالرَّبَذَةِ .