Musnad Ahmad 1174:0

Narrated to us by Yahya ibn Sa’id, from Hisham, who said: My father informed me, from Salamah bint Ma’qal, that she came to the Prophet (ﷺ) and said: "O Messenger of Allah, what is the ruling on a woman who is pregnant from adultery?" He (ﷺ) said: "She is not to be stoned while she is pregnant, nor is she to be struck, nor is she to be harmed. Wait until she gives birth." When she gave birth, she came to him, and he said: "Now, go and stone her." So she was stoned.