Abu Nasr al-Sulami said that he met Sayyiduna Ali (may Allah be pleased with him), who had entered ihram for both Hajj and Umrah, while he himself had entered ihram for Hajj. He said: I said to Ali: Shall I enter ihram for both together? Ali (may Allah be pleased with him) said: That would only have been if, when you began, you had called for your vessel of water and bathed, then entered ihram for both together, then performed two tawafs, one tawaf for your Hajj and one tawaf for your Umrah; then nothing would become lawful for you until the Day of Sacrifice.
Al-Thawri narrated it from Mansur, and in it (Ali) said: No, that would have been if you had begun with Umrah. Ali (may Allah be pleased with him) said: When you combine (Hajj and Umrah), do such-and-such, and he mentioned it with the same meaning. Mansur was unsure whether he heard it from Malik himself or from Ibrahim from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8749