Musnad Ahmad 303:14

'Abd al-Rahman ibn Mahdi narrated to us, he said: Hammad ibn Salamah narrated to us, from Thabit, from Mutarrif, from his father, who said: "I arrived at the Messenger of Allah (peace and blessings of Allah be upon him) while he was praying, and from his chest was a rumbling like the rumbling of a boiling pot."