Reference : Hisn al-Muslim 203

اللّهُـمَّ إِنّـي أَعـوذُبِكَ أَنْ أُشْـرِكَ بِكَ وَأَنا أَعْـلَمْ، 
وَأَسْتَـغْفِرُكَ لِما لا أَعْـلَم
Allāhumma innī a`ūdhu bika an ushrika bika wa anā a`lam, 
wa astaghfiruka limā lā a`lam.


O Allah, I seek refuge in You lest I associate anything with You knowingly, 
and I seek Your forgiveness for what I know not.

Reference:
Ahmad 4/403. See also Al-Albani, Sahihul-Jami' As-Saghir 3/233 and 
Sahihut-Targhib wat- Tarhib 1/19.

Chapter: For fear of shirk