Tawus (may Allah have mercy on him) says: I heard Sayyiduna Ibn al-Zubayr (may Allah be pleased with them both) saying: Al-i'rab is not permissible for the muhrim. So I said to Sayyiduna Ibn Abbas (may Allah be pleased with them both): What is al-i'rab? He said: "Sexual intercourse."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9172