What is the earliest explicit mention of infant baptism? The Bible describes several adults being baptized (notably Jesus and the Ethiopian eunuch that Phillip baptized). And while it mentions many others being baptized, it doesn't explicitly mention infants (although supporters of infant baptism do have a biblical basis).

What is the earliest explicit mention in historical documents of infants being baptized? I'm guessing that such a mention might take the form of a specific reference to an individual or group of infants being baptized, or something like instructions for the process of baptizing infants.

(To clarify, I am looking for explicit references to infant baptism outside of the Bible; there is considerable debate over whether the scriptures support infant baptism or if they only allowed for credobaptism alone. All of that is more suited to other questions on this site).
What is the earliest explicit mention of infant baptism?
It seems that St. Irenaeus (202 AD) is the earliest to reference to the practice of infant baptism. In 185 AD, he wrote the following:

Irenaeus, c. A.D. 185:

He came to save all through means of Himself—all … who through Him are born again* to God—infants, and children, and boys, and youths, and old men. He therefore passed through every age, becoming an infant for infants, thus sanctifying infants; a child for children, thus sanctifying those who are of this age, being at the same time made to them an example of piety, righteousness, and submission … (Against Heresies II:22:4)

*Note here that the term &quot;born again&quot; was synonymous with baptism to early Christians. That really didn't change until the time of the pietists in the 17th century. - Infant Baptism

There is no argument against infant baptism in the Early Church although Tertullian himself favored believer's baptism.

One argument in support of the baptism of infants comes from the fact that controversy over the practice is conspicuously absent from the history of the early church. There is no question that Origen was baptized as an infant in 180 A.D., just 80 years after the death of the last Apostle, John the Evangelist. There are other possible references to infant baptism at earlier dates, but these references are somewhat unclear in their meaning. Born in the mid fourth century (358 A.D.), Augustine wrote, &quot;This doctrine is held by the whole church, not instituted by councils, but always retained.&quot; Tertullian (ca. 155-230) did argue in favor of believer's baptism, but this was in reaction to the un-biblical practice that was being taught in some areas that Christians should wait to be baptized until just before death, and not in reaction to infant baptism. Francis Schaeffer argued, &quot;Those who would teach that the practice of the early Church was not infant baptism should be able to show in Church History when it started. There is no such break recorded.&quot; - Infant baptism

For more on infant baptism and the Church Fathers one can read this:

Irenaeus
“He [Jesus] came to save all through himself; all, I say, who through him are reborn in God: infants, and children, and youths, and old men. Therefore he passed through every age, becoming an infant for infants, sanctifying infants; a child for children, sanctifying those who are of that age . . . [so that] he might be the perfect teacher in all things, perfect not only in respect to the setting forth of truth, perfect also in respect to relative age” (Against Heresies 2:22:4 [A.D. 189]).
“‘And [Naaman] dipped himself . . . seven times in the Jordan’ [2 Kgs. 5:14]. It was not for nothing that Naaman of old, when suffering from leprosy, was purified upon his being baptized, but [this served] as an indication to us. For as we are lepers in sin, we are made clean, by means of the sacred water and the invocation of the Lord, from our old transgressions, being spiritually regenerated as newborn babes, even as the Lord has declared: ‘Except a man be born again through water and the Spirit, he shall not enter into the kingdom of heaven’ [John 3:5]” (Fragment 34 [A.D. 190]).
Hippolytus
&quot;Baptize first the children, and if they can speak for themselves let them do so. Otherwise, let their parents or other relatives speak for them&quot; (The Apostolic Tradition 21:16 [A.D. 215]).
Origen
“Every soul that is born into flesh is soiled by the filth of wickedness and sin. . . . In the Church, baptism is given for the remission of sins, and, according to the usage of the Church, baptism is given even to infants. If there were nothing in infants which required the remission of sins and nothing in them pertinent to forgiveness, the grace of baptism would seem superfluous” (Homilies on Leviticus 8:3 [A.D. 248]).
“The Church received from the apostles the tradition of giving baptism even to infants. The apostles, to whom were committed the secrets of the divine sacraments, knew there are in everyone innate strains of [original] sin, which must be washed away through water and the Spirit” (Commentaries on Romans 5:9 [A.D. 248]). - The Church Fathers: Infant Baptism

For those interested the following question may be helpful: Has the term “born again” always been synonymous with baptism with the Christians of the Early Church?
Use the &quot;Comment&quot; button if you can find a reliable, accurate reference to an original pre-17th century document that separates the term &quot;born again&quot; from baptism. I'd be happy to see it.