Musnad Ahmad 182:1

Ya'qub narrated to us, he said: I heard my father narrating from Yazid, meaning Ibn al-Had, from Muhammad ibn Ibrahim ibn al-Harith, from Suhayl ibn Bayda', that he said: The Messenger of Allah, may Allah bless him and grant him peace, called out one night while I was riding behind him: "O Suhayl ibn Bayda'," raising his voice with it repeatedly, until those behind us and in front of us heard, so they gathered, and they knew that he intended to speak about something. "Indeed, whoever says: 'There is no god but Allah' (La ilaha illa Allah), Allah the Almighty and Majestic makes Paradise obligatory for him by it, and frees him from the Fire by it."