Musannaf Ibn Abi Shayba 3:3821

Ya'la ibn 'Ubayd narrated to us, from Muhammad ibn Ishaq, from Sa'id ibn Abi Hind, who said: I sent a servant of mine, who was a scribe and an accountant, to Sa'id ibn al-Musayyib to ask him about praying in a qamis (tunic) without an izar (lower garment) underneath it. He said: "There is no harm in it, provided it does not reveal what is underneath it."