Musnad Abu Ya'la al-Mawsili 111:136

Abu Khaythama narrated to us, Sufyan narrated to us, from Ibrahim ibn Maysara, from Ubayd ibn Sa'd, attributing it to the Prophet (may Allah bless him and grant him peace), who said: "Whoever loves my natural disposition (fitra) should follow my Sunnah, and from my Sunnah is marriage."