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
Hadith No. 300
Sunan Ibn Majah
Gradings:
Al-Albani:
Sahih
Muhammad Fouad Abd al-Baqi:
Sahih
Zubair Ali Zai:
Isnaad Sahih