Where is it stated that candidates in the RCIA process should receive the sacrament of reconciliation before the Vigil?

The Rite of Christian Initiation of Adults, Part II, Chapter 4 gives the instructions for the preparation of uncatechized adults for Confirmation and Eucharist and both those who were baptized Catholic and those who were validly baptized in another Christian tradition.

#408 states: During the Lenten season penitential services should be arranged in such a way as to prepare these adults for the celebration of the sacrament of penance.
#482 states: If the profession of faith and reception take place within Mass, the candidate, according to his or her own conscience, should make a confession of sins beforehand, first informing the confessor that he or she is about to be received into full communion.

In the National Statutes for the Catechumenate at the back of the RCIA book:
#27 and #36 state: The celebration of the sacrament of reconciliation with candidates for reception into full communion [and candidates for Confirmation and Eucharist] is to be carried out at a time prior to and distinct from the celebration of the rite of reception. As part of the formation of such candidates, they should be encouraged in the frequent celebration of this sacrament.