He traced the hadith back until it reached Abu Saeed, who said that this was mentioned in the presence of the Prophet ﷺ, who asked, "What is that you are speaking of?"They replied, "A man has a wife who is nursing, and he has relations with her but does not wish her to conceive from him. And a man has a slave girl, and he has relations with her but does not wish her to conceive from him."He said, "There is no harm upon you if you do that, for it is only the decree."Ibn Awn said, "I related this to al-Hasan, and he said, 'No harm upon you, as if this were a rebuke.'"
حدثنا إسماعيل أخبرنا ابن عون عن محمد
فرد الحديث حتى رده إلى أبي سعيد قال ذكر ذلك عند النبي ﷺ فقال وما ذاكم قالوا الرجل تكون له المرأة ترضع فيصيب منها ويكره أن تحمل منه والرجل تكون له الجارية فيصيب منها ويكره أن تحمل منه فقال فلا عليكم أن تفعلوا ذاكم فإنما هو القدر قال ابن عون فحدثت به الحسن فقال فلا عليكم لكأن هذا زجر
Its isnad is sahih. Muhammad is Ibn Sirin, a trustworthy imam, and Abdul-Rahman bin Bishr bin Masud Ansari is trustworthy. The hadith is found in Muslim 2/163 no. 1438, in the book of marriage, on the ruling of coitus interruptus, and in Nasai 6/108 no. 3327 with similar wording, and in many other sources besides these.