It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) sent Abu Bakr (may Allah be pleased with him) with Surah Bara'ah (al-Tawbah), then sent Ali (may Allah be pleased with him) after him, who took it. Abu Bakr (may Allah be pleased with him) asked: Has something happened concerning me? He (peace and blessings be upon him) said: "No. You are my companion in the cave, and at the Pond as well; and no one will deliver on my behalf except me or Ali."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12075