The Prophet (may Allah's peace and blessings be upon him) did not take a second wife until Khadījah died
Authentic hadithعن عائشة رضي الله عنها ، قالت: «لم يتزوج النبي صلى الله عليه وسلم على خديجة حتى ماتت».
‘Ā'ishah (may Allah be pleased with her) reported: The Prophet (may Allah's peace and blessings be upon him) did not take a second wife until Khadījah died.
Explanation
The Prophet (may Allah's peace and blessings be upon him) did not get married while Khadījah bint Khuwaylid (may Allah be please with her) was his wife until after she died. This attitude implies how much the Prophet (may Allah's peace and blessings be upon him) cherished Khadījah, his first wife.
Source · Narrated by Muslim
From the Hadith Encyclopedia (hadeethenc.com).