Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: A man came to the Prophet (peace and blessings be upon him) and said: I had intercourse with my wife in Ramadan. He (peace and blessings be upon him) said: "Free a slave." He said: I cannot find one. He (peace and blessings be upon him) said: "Fast two consecutive months." He said: I am not able to. He (peace and blessings be upon him) said: "Then feed sixty poor people." He said: I do not have the means. A basket containing fifteen sa' of dates was brought to him (peace and blessings be upon him). He (peace and blessings be upon him) said: "Take this and feed it on your own behalf." He said: O Messenger of Allah (peace and blessings be upon him)! There is no household between these two mountains more in need than us. He (peace and blessings be upon him) said: "Take it and feed your family."
And the basket contained fifteen sa' of dates.
It seems most likely that the estimate of the basket as fifteen sa' comes from a narration of Amr bin Shu'ayb, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8042