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 What is the earliest explicit mention in historical documents of infants being baptized?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).