Musnad Ahmad 31:91

Isma'il narrated to us, Ibn 'Awn informed us, from Muhammad, from 'Abd al-Rahman ibn Bishr ibn Mas'ud, who said: He traced the hadith until he attributed it to Abu Sa'id. He said: This was mentioned in the presence of the Prophet, may Allah bless him and grant him peace, and he said: "What is that?" They said: A man has a wife who is breastfeeding, and he has intercourse with her, but he dislikes that she should become pregnant by him. And a man has a slave woman, and he has intercourse with her, but he dislikes that she should become pregnant by him. He said: "There is no harm upon you if you do that, for it is only the decree." Ibn 'Awn said: So I narrated it to al-Hasan, and he said: "There is no harm upon you," as if this were a discouragement.