Yusuf bin Abi Burdah narrated:
"I heard my father say: 'I entered upon 'Aishah, and I heard her say: "When the Messenger of Allah exited the toilet, he would say: Ghufranaka (I seek Your forgiveness).'" (Sahih)Another chain with similar wording. (FR)
English reference (FR): Vol. 1, Book 1, Hadith 300 (FR)
Arabic reference (FR): Book 1, Hadith 318 (FR)