(a) It is narrated from Zuhrah: We were sitting with Zayd bin Thabit (may Allah be pleased with him). They sent some people to Usamah bin Zayd (may Allah be pleased with him), and they asked him about the middle prayer. He said: It is Zuhr; the Messenger of Allah (peace and blessings be upon him) used to offer it at midday (in the heat).
(b) In a narration transmitted through another chain there is this addition: "That is, Zayd (may Allah be pleased with him) said: It is the Zuhr prayer," and then they sent this message to Usamah (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2155