It is narrated from Ubayy bin Ka'b (may Allah be pleased with him) that he used to say: Whoever has not ejaculated, ghusl is not obligatory upon him. Then, before his death, Ubayy bin Ka'b (may Allah be pleased with him) retracted from this statement.
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 100
Hadith Takhrijاسناده ضعیف بسبب ضعف شيخ الشافعی، لكنه صح من غير هذا الطريق اخرجه عبدالرزاق: (960) وابن أبي شيبة: (949) والبيهقي: 1/ 166