It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that when the Zuhr prayer was started for the Messenger of Allah (peace and blessings be upon him), a person would go to al-Baqi', relieve himself and come back, then perform ablution and come, and the Messenger of Allah (peace and blessings be upon him) would still be in the first rak'ah, which he (peace and blessings be upon him) used to lengthen somewhat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2489