jazaakallah

Jazaakallah

Q: This is with regards to saying Jazaakumullah khair. One of my beloved teachers told us that it is preferable to say: Jazaakumullah khair i.e. plural form instead of Jazaakallah khair or Jazaakillah khair. Kindly explain me with acceptable evidence if it is correct to do so as my friends are not accepting this.