Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates from Sayyiduna Umar (may Allah be pleased with him) about a man who made zihar from his four wives with a single statement; he said: Only one expiation is due from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15253