Where can anyone cite credible proof that she said it?
Of course, it’s not magisterial teaching that one must receive in the hand. It is, however, a discipline of the Church that is protected by a negative infallibility, ie, the Church cannot approve a discipline that will lead the faithful into impiety.
As for the rosary, that’s two seperate things. “You should say the rosary daily” (a prayer commended by the Church to the faithful) and “you should only receive on the tongue” (regarding a practice permitted by the Church, a liberty she grants) are distinctly different. The former is just commending a particular devotion, the latter is an attempt to constrain the conscience of the faithful where the Church has given them the liberty to choose. The latter implies that the Church is wrong.
There’s one way to settle this: recieve on the tongue if it pleases you to do so. I do, as an act of reparation for the disunity among the faithful. But I don’t believe Mother Teresa said it (I’ve never seen it reported by a credible source, ie, outside radical traditionalist sites) and I don’t believe the Blessed Mother of God opposes it (indeed, several sites that come up when you google “Mother Teresa and communion in the hand” mention the Bayside apparitions, which are NOT approved by the Church).