Musannaf Ibn Abi Shayba 9:1049

Waki’ narrated to us, from Shu’bah, from al-Hakam, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) married Umm Salamah, he stayed with her for three [days]. He said: "If you wish, I will stay with you for seven [days], and if you wish, I have already stayed seven [days] for others." It was said to al-Hakam: "Who narrated this hadith to you?" He said: "This hadith is well-known among the people of the Hijaz."