Musnad Ahmad 27:1903

'Abd al-Razzaq narrated to us, from Ma'mar, from al-Zuhri, from Salim, or from more than one, he said: Ibn 'Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) and Ubayy ibn Ka'b set out, coming to the palm trees in which Ibn Sayyad was. When they entered the palm trees, the Messenger of Allah (peace and blessings of Allah be upon him) began to take cover behind the trunks of the palm trees, and he was trying to catch Ibn Sayyad unawares, so that he might hear something from Ibn Sayyad before he saw him. Ibn Sayyad was lying on his bed, wrapped in a velvet blanket of his, in which there was a murmuring sound. His mother saw the Messenger of Allah (peace and blessings of Allah be upon him) while he was taking cover behind the trunks of the palm trees, so she said: "O Saf" — and that was his name — "this is Muhammad." So he jumped up. The Messenger of Allah (peace and blessings of Allah be upon him) said: "If she had left him alone, he would have made it clear."