Musnad Ahmad 162:1

Qutaybah ibn Sa'id narrated to us, he said: Bakr ibn Mudar informed us, from Ibn al-Had, from Muhammad ibn Ibrahim, from Sa'id ibn al-Salt, from Suhayl ibn al-Bayda', who said: While we were on a journey with the Messenger of Allah, peace and blessings of Allah be upon him, and I was riding behind him, the Messenger of Allah, peace and blessings of Allah be upon him, said: "O Suhayl ibn al-Bayda'," and he raised his voice two or three times, and each time Suhayl answered him. The people heard the voice of the Messenger of Allah, peace and blessings of Allah be upon him, and they thought that he wanted them, so those who were in front of him stopped, and those who were behind him caught up to him. When they had gathered, the Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, whoever testifies that there is no god but Allah, Allah has forbidden him to the Fire and made Paradise obligatory for him."