Abd al-Malik says that Ata was asked: "When should the one performing Umrah stop the talbiyah?" He said: "Ibn Umar (may Allah be pleased with them both) says: When he enters the Haram. And Ibn Abbas (may Allah be pleased with them both) says: When he touches the Black Stone." I said: "O Abu Muhammad, which of the two do you prefer?" He said: "The view of Ibn Abbas (may Allah be pleased with them both)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9409