I am not aware of the story mentioned in your question. Neither do I have any knowledge of a specific Dua of Anas Bin Malik (R.A) except that the Holy Prophet Sallallahu Alaihe Wa Sallam prayer for him by saying "O' Allah place Blessings in the wealth and progeny of Anas" and therefore he had many many children as a result of the Duaa of the Holy Prophet Sallalahu Alaihe Wa Sallam for him.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America
12 Shawwal 1431/ September 21 2010