Reference : Hisn al-Muslim 11

غُفْـرانَك
Ghufrānaka

I seek Your forgiveness.

Reference:
Abu Dawud, Ibn Majah and At-Tirmidhi. An-Nasa'i recorded it in 'Amalul-Yawm 
wal-Laylah. Also see the checking of Ibn Al-Qayyim's Zadul-Ma'ad, 2/387.

Chapter: After leaving the bathroom