Nicene and Post-Nicene Fathers, Ser. II, Vol. III:
Life and Works of Rufinus with Jerome's Apology Against Rufinus.: Section 23

23. It is written that when the side of Jesus was pierced “He shed thereout blood and water.” 3332 This has a mystical meaning. For Himself had said, “Out of His belly shall flow rivers of living water.” 3333 But He shed forth blood also, of which the Jews sought that it might be upon themselves and upon their children. He shed forth water, therefore, which might wash believers; He shed forth blood also which might condemn unbelievers. Yet it might be understood also as prefiguring the twofold grace of baptism, one that which is given by the baptism of water, the other that which is sought through martyrdom in the outpouring of blood, for both are called baptism. But if you ask p. 553 further why our Lord is said to have poured forth blood and water from His side rather than from any other member, I imagine that by the rib in the side the woman is signified. Since the fountain of sin and death proceeded from the first woman, who was the rib of the first Adam, the fountain of redemption and life is drawn from the rib of the second Adam.

Footnotes

John xix. 34

John vii. 38