Sahih Muslim 1187
Sahih Muslim · Book 15: The Book of Pilgrimage · Clarifying that it is better to enter Ihram when a person's mount sets off with him, heading towards Makkah, not straight after the two rak'ah
'Abdullah b. 'Umar reported:
وَحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى ، أَخْبَرَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي يُونُسُ ، عَنِ ابْنِ شِهَابٍ ، أَنَّأَخْبَرَهُ أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ - رضى الله عنهما - قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم رَكِبَ رَاحِلَتَهُ بِذِي الْحُلَيْفَةِ ثُمَّ يُهِلُّ حِينَ تَسْتَوِي بِهِ قَائِمَةً .
I saw the Messenger of Allah (ﷺ) riding on his camel at Dhu'l-Hulaifa and pronouncing Talbiya as it stood up with him.