In another narration from Ibn al-Musayyib (may Allah have mercy on him), fifteen sa' are mentioned without doubt. Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: We were with the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) mentioned the man who had had intercourse with his wife. In it is: "Feed sixty poor persons." He said: "I cannot." A basket of dates containing fifteen sa' was brought to him (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Take it and feed sixty poor persons."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19970