Were Prophet Mohammad, sallallaahu 'alayhi wa sallam, and Khadijah bint Khuwaylid related before getting married?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Khadeejah bint Khuwaylid was definitely related to the Prophet, sallsllaahu ʻalayhi wa sallam, since they both belonged to the same tribe, Quraysh. The lineage of Khadeejah was detailed in Seerat Ibn Hishaam and other books as follows, "Khadeejah bint Khuwaylid ibn Asad ibn Abd Al-ʻUzza ibn Qusayy ibn Kilaab ibn Murrah ibn Kaʻb ibn Lu'ayy ibn Ghaalib ibn Fihr."

She was from the Banu Asad clan of the Quraysh tribe. The lineage of the Prophet, sallallaahu ʻalayhi wa sallam, meets hers in Qusayy ibn Kilaab, who was the fifth grandfather of the Prophet, sallallaahu ʻalayhi wa sallam, and the fourth grandfather of Khadeejah .

Allaah knows best.