Abu Mijlaz (may Allah have mercy on him) said regarding "But do not make promises to them secretly, except that you speak an honourable word" [Surah al-Baqarah: 235]: "'Secretly' (al-sirr) means zina." He (the narrator) said: Then I also asked al-Hasan (may Allah have mercy on him) about it, and he said: "It means zina."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14026