The Prophet ﷺ passed by Saad while he was making supplication, and he said, "One, one."
حدثنا عبد الله بن محمد قال عبد الله بن أحمد وسمعته أنا منه حدثنا حفص بن غياث عن الأعمش عن أبي صالح
أن النبي ﷺ مر بسعد وهو يدعو فقال أحد أحد
Its isnad is sahih. Abu Dawud narrated it, 2/80, no. 1499, in the Book of Prayer, chapter on supplication, and Tirmidhi, 5/557, no. 3557, in the Book of Supplications, chapter on the supplication of the Prophet, and Nasai, 3/38, no. 1272, in the Book of Forgetfulness, chapter on the prohibition of gesturing with two fingers.