Musnad Ahmad 1017:103

Husayn ibn Muhammad narrated to us, Khalaf—meaning Ibn Khalifah—narrated to us, from Abu Janab, from Sulayman ibn Buraydah, from his father, that the Messenger of Allah (peace and blessings of Allah be upon him) went out on the expedition of the Conquest. He went out walking to the graves, and when he reached the nearest of them, he sat by it as if he were speaking to a person. He sat weeping. Umar ibn al-Khattab met him and said, "What makes you weep? May Allah make me a ransom for you." He said, "I asked my Lord to permit me to visit the grave of the mother of Muhammad, and He permitted me. Then I asked Him to permit me to seek forgiveness for her, but He refused. I had forbidden you from three things: from the meat of the sacrificial animals, that you should keep it after three days—so eat whatever seems good to you; and from visiting the graves—so whoever wishes, let him visit, for I have been permitted to visit the grave of the mother of Muhammad, and whoever wishes, let him call upon [Allah]; and from the vessels in which you drink: the gourd, the glazed jar, and the pitch-coated vessel. I had commanded you regarding vessels, but a vessel does not make anything lawful or unlawful. So avoid every intoxicant."