(a) Abu Sa'id (may Allah be pleased with him) narrates that Sayyiduna Ubayy bin Ka'b and Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with them both) differed about praying in a single garment. Sayyiduna Ubayy (may Allah be pleased with him) said that it may be prayed in one garment, while Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) held that it should be two garments. Sayyiduna Umar (may Allah be pleased with him) passed by them, reproached them both and said: "It truly grieves me that two Companions of Muhammad (peace and blessings be upon him) should differ about one and the same thing. By which of your two fatwas are the people to go? As for Ibn Mas'ud (may Allah be pleased with him), he did not fall short, but the correct view is what Ubayy (may Allah be pleased with him) said."
(b) This narration was reported by Abu Mas'ud al-Jurayri through Abu Nadrah, and in it there is no mention of Sayyiduna Umar (may Allah be pleased with him). In it, Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "That was only when garments were scarce; but when Allah Almighty has given abundance, praying in two garments is better."
(c) This and the narration before it both indicate that what Sayyiduna Umar and Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with them both) ordered, namely praying in two garments, is recommended (mustahabb), not obligatory (wajib).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3285