Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said concerning this saying of Allah Almighty, "And (also forbidden are) married women, except those whom your right hands possess" [Surah al-Nisa: 24]: Intercourse with any woman who has a husband is zina (fornication), except for a slave-woman.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13955