It is enough to recite the Dua one time. You can also repeat it.
Dua is in the famous Hisnul Muslim collection:
http://hisnulmuslim.readismed.com/zik...

Referenz: Al-Bukhari 11/116 (Hadith no. 6316), Muslim 1/526 (Hadith no. 763).

Recited by Yahya Hawwah