Musannaf Ibn Abi Shayba 22:196

Muhammad ibn Ubayd narrated to us, from al-Awwam ibn Hawshab, from Ibrahim al-Taymi, that Abu Bakr was asked about the [meaning of the word] "Abba" (أبا) in the verse: "And fruits and Abba" (وَفَاكِهَةً وَأَبًّا). He said: "Which sky would shade me and which earth would carry me if I were to say regarding the Book of Allah that which I do not know?"