Sahih Bukhari Volume 007, Book 072, Hadith Number 797.
Narrated By Ibn 'Umar : I heard Allah's Apostle, while he was in the state of lhram and his hair was stuck together with gum, saying, "Labbaik, Allahumma Labbaik, Labbaik La Shanka laka Labbaik. Inn-al-Hamda Wan-Ni'mata Laka wal-Mulk, La Shanka Lak." He did not add anything to those words. (See Hadith No. 621, Vol. 2)