Asim bin Kulayb said that his father informed him that Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) said: I thought (to myself): I will certainly watch the Messenger of Allah (peace and blessings be upon him) to see how he performs the prayer. So I looked at him: he stood up, said the takbir and raised his hands. Then he mentioned the complete hadith. At its end he said: Then after that I came in the winter season, and I saw that the people had wrapped themselves in large cloaks, but their hands were moving (when raising them) beneath those cloaks.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2321
Sufyan bin Uyaynah narrates from Asim that Sayyiduna Wa'il (may Allah be pleased with him) said: Then I came to them in the winter season, and I saw that they raised their hands (when doing raf' al-yadayn) inside their cloaks.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2322