Musnad Abu Ya'la al-Mawsili 1:100

Abu Khaythama narrated to us, Jarir ibn Abd al-Hamid narrated to us, from Mansur, from Abu Wa'il, who said: It was narrated to me that Abu Bakr met Talha and said, "Why do I see you looking distressed?" He said, "A word I heard from the Messenger of Allah (may Allah bless him and grant him peace), he claims that it is a guarantor [of salvation], but I did not ask him about it." Abu Bakr said, "I know what it is." He said, "What is it?" He said, "There is no god but Allah."