Musnad Ahmad 460:5

And he mentioned [it] attached to it. He said: The Messenger of Allah, may Allah bless him and grant him peace, sat in a gathering, and Gabriel, peace be upon him, came to him and sat before the Messenger of Allah, may Allah bless him and grant him peace, and he mentioned the hadith, and he said in it: "If you wish, shall I tell you of signs for it other than that?" He said: "Yes, O Messenger of Allah, so tell me." The Messenger of Allah, may Allah bless him and grant him peace, said: "When you see the slave-woman giving birth to her mistress." And he mentioned the hadith.