It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings of Allah be upon him) said: No person should have intercourse with a woman who is pregnant from another man.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8814
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لضعف رشدين