Musnad Ahmad 28:473

Rawh narrated to us, Hammad narrated to us, from Thabit, from Shu’ayb ibn ‘Abdullah ibn ‘Amr, from his father, that the Prophet, peace and blessings of Allah be upon him, said to him: "Fast one day, and you shall have [the reward of] ten days." He said: "Increase me, O Messenger of Allah, for I have strength." He said: "Fast two days, and you shall have [the reward of] nine days." He said: "Increase me, for I find strength." He said: "Fast three days, and you shall have [the reward of] eight days."