Musnad Ahmad 1153:335

Waki’ narrated to us, Sufyan narrated to us, from Mansur, from Musa ibn ‘Abdullah ibn Yazid al-Khatmi, from a freed slave of ‘Aishah, from ‘Aishah, who said: "I never looked at the private part of the Prophet, may Allah bless him and grant him peace, at all," or "I never saw the private part of the Prophet, may Allah bless him and grant him peace, at all."