Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "When one of you wishes to return to his wife again, let him perform ablution, for it makes him more energetic."
If al-Shafi'i (may Allah have mercy on him) meant this hadith, then its chain is sahih (authentic); perhaps he did not come across its chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14088