Whenever the Prophet ﷺ stood for prayer, he would raise his hands fully extended.
حدثنا حسين بن محمد قال أنبأنا ابن أبي ذئب عن محمد بن عمرو بن عطاء عن محمد بن عبد الرحمن بن ثوبان
أن النبي ﷺ كان إذا قام إلى الصلاة رفع يديه مدا
Its isnad is sahih. Muslim narrated something similar to it, 1/293, no. 390, in the Book of Prayer, chapter on the recommendation of raising the hands level with the shoulders. Abu Dawud narrated it, no. 721, in the chapter on the opening statement, raising the hands. Abdul-Razzaq narrated it, 2/67, no. 2517, in the Book of Prayer, chapter on the opening takbir. And Tabarani narrated it in Al-Kabir, 19/284.With similar wording, it was also narrated by Tirmidhi, 2/6, no. 240, in the Book of Prayer, chapter on raising the fingers at the takbir, and by Nasai, 2/121, no. 876.