It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would command whoever accepted Islam to be circumcised."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4889
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[طب] عن قتادة الرهأوي. الضعيفة ٤٢٦٠.