Musnad Ahmad 1156:151

Yahya ibn Sa'id al-Umawi narrated to us, he said: Ibn Jurayj informed us, from Habib ibn Abi Thabit, from 'Abd al-Hamid ibn 'Abd Allah, and al-Qasim ibn 'Abd al-Rahman ibn al-Harith ibn Hisham, from Abu Bakr ibn al-Harith ibn Hisham, from Umm Salamah, that the Prophet, peace and blessings of Allah be upon him, said to her: "If you wish, I will stay with you for seven [days], and if I stay with you for seven, I will stay with my wives for seven."