Al-Harith bin Amirah al-Harithi narrates that Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) was struck by the plague, and he kept sending al-Harith bin Amirah to Abu Ubaydah bin al-Jarrah to ask how he was. Abu Ubaydah showed him a plague sore that was on his palm; its matter seemed very grave to al-Harith, and when he saw it he became frightened. So Abu Ubaydah swore to him by Allah that he would not love to have red camels in exchange for it.
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 361