The Prophet ﷺ used to observe a brief pause in the prayer.
حدثنا وكيع عن سفيان عن عمارة بن القعقاع عن أبي زرعة
أن النبي ﷺ كان له سكتة في الصلاة
No references are available.
Its isnad is sahih. Abu Dawud narrated it, 778, in the Book of Prayer, on the pause at the opening, and Nasai narrated it in the Book of the Opening, on the imam's silence after his opening, 2/128, no. 894.