TriploySunni, With all due respect, but no, it was not "quoted from a Shi'a site". Either way, since there are no authentic narrations proving conclusively that Mu'awiya (ra) was behind it, it is not permissible to state such, unless, of course, one can provide sufficient proof. But any attempt to do so will most likely fail miserably. And implicating anyone in such a hideous crime, let alone a Sahabi of Rasulu'Allah (salla'Allahu alayhi wa alihi wa sallam), without solid proof is prohibited. As for Yazid, then there is a case for that, although not necessarily conclusive. All that can be said for sure is that Al-Hasan (ra) was poisoned by his wife Ja'da bint Al Ash'ath, and that she had been promised wealth and marriage to Yazid. Who made those promises? We cannot say for sure. Wa Allahu a'lam