I have read it somewhere and can't remember now. Buddha told Bhikkus that he never admires a disiple who stops his effort after attaining "sotapanna" state. He should still work hard until he is completely clean and attains Arhat status.
The thing you are looking for is probably a misinterpretation of this passage from Maha-Parinibbana Sutta:
... among these five hundred bhikkhus even the lowest is a stream-enterer, secure from downfall, assured, and bound for enlightenment."
- And the Blessed One addressed the bhikkhus, saying: "Behold now, bhikkhus, I exhort you: All compounded things are subject to vanish. Strive with heedfulness!"
Handa dani bhikkhave amantayami vo: Vayadhamma sankhara appamadena sampadetha.
And this one in regards to Samsara:
“Just as even a little dung stinks, so I do not recommend even a little becoming, not for so much as a finger snap”