Sayyiduna Anas bin Malik (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) would come out and lead them in the Zuhr prayer when the sun had declined.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1240
(Commentary on Hadith 1239) From this hadith, it is understood that the time for the Zuhr prayer begins immediately after the sun passes its zenith (zawal al-shams).