The Prophet (peace and blessings be upon him) said:
"A minhah (an animal lent out for use) is to be returned, and people are bound by their conditions so long as they are in accordance with the truth."
Hadith Referenceصحیح الجامع الصغیر / حرف الميم / 6732
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[البزار] عن أنس. الصحيحة ٦١٠و ٦١١، الإرواء ١٣٠٣.