Hadith 31-145

وَبِهذَا الإِسنادِ عَنْ الحَسَنِ، عَنْ عَلِيٍ‏عَلَيْهِمَا السَّلاَمُ أَنَّهُ سُمّى حَسَناً يَوْمَ السّابِعِ وَاشتُقَّ مِن اسمِ الحَسَنِ حُسَيْناً، وَذَكَرَ أَنَّهُ لَمْ يَكُنْ بَيْنَهُما إِلّا الحَمْلُ.

According to the same documentation, on the authority of Al-Hassan ibn Ali (a.s.), on the authority of Ali (a.s.), “They named my son Al-Hassan on the seventh day (after his birth). Al-Husayn’s name was derived from Al-Hassan. There was not any more than one period of pregnancy difference between their birth.”