Abdullah bin Umar (may Allah be pleased with them both) said regarding the saying of Allah Almighty, "So whoever makes Hajj obligatory upon himself in them" [Surah al-Baqarah: 197]: making it obligatory means saying the talbiyah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8715