Musnad Ahmad 31:492

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from al-Aswad ibn Qays, from Nabih, from Abu Sa’id al-Khudri, that they "went out with the Messenger of Allah, peace and blessings of Allah be upon him, on a journey. They descended in groups; a group with so-and-so, and a group with so-and-so." He said: "I descended in the group of Abu Bakr. There was with us a Bedouin from the people of the desert. We descended upon a household of the Bedouins, and among them was a pregnant woman. The Bedouin said to her: 'Would it please you if you gave birth to a boy? If you give me a sheep, you will give birth to a boy.' So she gave him a sheep, and he rhymed some rhymes for her." He said: "So he slaughtered the sheep. When the people sat down to eat, a man said: 'Do you know what this sheep is?' And he informed them. He said: 'So I saw Abu Bakr going out to relieve himself, trying to induce vomiting, and vomiting.'"