It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) had a large bowl which had four rings attached to it."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4828
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[طب] عن عبد الله بن بسر. الصحيحة ٢١٠٥: أبو الشيخ.