Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 916
Abu Sa'id al-Khudri reported: The noon prayer would start and one would go to al-Baqi' and after having relieved himself he would perform ablution and then come, while the Messenger of Allah (may peace be upon him) would be in the first rak'ah, because he would prolong it so much.