Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Circumcision in a dream indicates following the Sunnah and purification from sins. For a polytheist, it is conversion to Islam, and for a sinner, repentance.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Circumcision indicates righteousness in religion and worldly life. Whoever sees himself circumcise himself is purifying himself from a sin.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Circumcision is interpreted as joy and happiness. Whoever sees himself circumcise a boy is striving for his righteousness.