Abu Nasr (may Allah have mercy on him) said: I met Sayyiduna Ali (may Allah be pleased with him), and he had entered ihram for Hajj and Umrah while I had entered it for Hajj only. I said: "Can I do as you have done?" He said: "That would be possible had you begun with Umrah." I said: "How should I do it if I want that?" He said: "Take a vessel of water and pour it over yourself, then say the talbiyah for both of them, then perform two tawafs for them and two sa'ys, and nothing forbidden will become lawful for you before the Day of Sacrifice (yawm al-nahr)."
This Abu Nasr is majhul (unknown). It has also been narrated from Ali (may Allah be pleased with him), both mawquf and marfu', through weak chains, which revolve around al-Hasan bin Umarah, Hafs bin Abi Dawud, Isa bin Abdullah and Hammad bin Abd al-Rahman; all of them are da'if (weak), and nothing they narrated on this is used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9427