Musnad Ahmad 30:3181

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "For every prophet there is a supplication with which he supplicates, and I wish to store my supplication as an intercession for my nation in the Hereafter." Ishaq said: "So I wish to store my supplication."