Abu al-Zubayr (may Allah have mercy on him) narrates: I asked Jabir (may Allah be pleased with him) about a divorced woman, whether she may perform i'tikaf. He said: No, nor the woman whose husband has died, until they have completed their waiting period ('iddah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8604