Didache 7:1
1 Regarding baptism, perform it like this: After explaining all these things, baptize in the Name of the Father, the Son, and the Holy Spirit, using running water. 2 If you don't have running water, use any other water, and if you can't use cold water, then use warm. 3 If you don't have either, pour water three times over the head in the Name of the Father, Son, and Holy Spirit. 4 Before the baptism, both the person conducting the baptism and the one being baptized should fast, along with anyone else who can. Also, instruct the person being baptized to fast for one or two days beforehand.
Mishnah Mikvaot 1:8
MishnahSuperior to such [water] is [the water of] the mikveh containing forty seahs, for in it people may immerse themselves and immerse other [things]. Superior to such [water] is [the water of] a spring whose own water is little but has been increased by a greater quantity of drawn water. It is equivalent to the mikveh in as much as it may render clean by standing water, and to an [ordinary] spring in as much as one may immerse in it whatever the quantity of its contents. Superior to them are "smitten waters" which can purify even when flowing [on the ground]. Superior to them are "living waters" for in them there is immersion for zavim and sprinkling for metzoraim, and they are valid for the preparation of the hatat waters.
Notes and References
"... The directives in Didache 7:1-3 with regard to the kinds of water to be used in baptism are clearly borrowed from Judaism ... various types of water are mentioned in a descending order from the best water, which is running water from a spring or river, to less and less good. 'Living water' has long been highly valued in Israel (compare Leviticus 14:5.50.52; Numbers 19:17) and in the Hellenistic-Roman world ... John the Baptist baptized in the Jordan (Matthew 3:6; Mark 1:5), and also from other early sources, we get the impression that the instruction to baptize in 'living water' in Didache 7:1 is archaic and goes back to the beginnings of the Christian mission (compare Acts 8:36; 16: 13). The earliest form of baptism seems to have been performed on the banks of a river, at a well or on the seashore. The Didache, however, embodies a concession toward this formerly strict practice. Should circumstances so demand, it permits performance of the rite of baptism 'in another kind of water.' Interestingly, the directives in Didache 7 seem to have their parallels in Jewish halakhic instructions about water for ritual washings. The normal condition of the water found in the rite of Christian baptism is indicated by the phrase 'living water', and also in the Mishnah it has the highest rank within the classification of kinds of water. In mishnah Mikvaot 1:1-8 a list is given classifying six kinds of water in an ascending order of value ..."
van de Sandt, H. W. M. Jewish Traditions in Early Christian Literature, Volume 5: The Didache (pp. 281-282) Brill, 2002