Hadith 31-19

وَبِهذَا الإِسنادِ قالَ: قالَ رَسُولُ اللَّهِ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ: اِختِنوا أَوْلادَكُم يَوْمَ السَّابِعِ؛ فَإِنَّهُ اَطهَرُ أَسرَعُ لِنَباتِ اللَّحْمِ.

According to the same documentation, God’s Prophet (S) said, “Circumcise your (male) children on the seventh day after birth as this would make them cleaner and speed up their growth.”