All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is confirmed in an authentic Hadeeth about Thikr (remembrance of Allaah) that the Prophet said at its end: "…And no one will get a reward like that of the one who did so [said so], except a person who did [said] more than that.”
We have not come across any Hadeeth with the wording that you asked about, but it is hoped that whoever multiplies the number then the mentioned reward will be multiplied for him as well, as the Favor of Allaah is vast.
However, a Muslim should preoccupy himself/herself by mentioning Allaah as much as possible and leaving the matter of reward to Allaah, The Most Generous, and The Grantor Who gives plentifully.
Allaah Knows best.