It is found in the first Khandhaka, the Mahakkhandhaka, the section on the ordination procedure. Here is the relevant passage (my translation):
Eunuchs
At one time a certain eunuch had gone forth as a monk. He approached the young monks and said, “Venerables, come and have sex with me.” The monks dismissed him, “Go away, eunuch. Who needs you?” He went to the big and fat novices, said the same thing, and got the same response. He then went to the elephant keepers and horse keepers, and once again he said the same thing. And they had sex with him.
They complained and criticized them, “These Sakyan ascetics are eunuchs. And those who are not have sex with them. None of them is celibate.” The monks heard their complaints. They told the Buddha and he said, “A eunuch should not be given the full ordination. If it has been given, he should be expelled.”
For why “eunuch” is a suitable translation for paṇḍaka, see the first reference given by @Gabriel_L above.