Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) that the Prophet (peace and blessings be upon him) named Sayyiduna Hasan (may Allah be pleased with him) on the seventh day, and the name of Husayn (may Allah be pleased with him) is also derived from Hasan; between the two of them (may Allah be pleased with them both) there was only the gap of one pregnancy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19295