Musnad Ahmad 32:1350

'Arim narrated to us, Mu'tamir narrated to us, he said: I heard my father narrating from Anas ibn Malik, who said: "The Prophet, may Allah bless him and grant him peace, confided a secret to me, and I did not inform anyone about it after him. Umm Sulaym asked me about it, but I did not inform her of it."