I passed by the Messenger of Allah ﷺ while he was praying, so I greeted him with salam, and he returned the greeting with a gesture. He said, "I only know that he gestured with his finger."
حدثنا حجاج بن محمد قال قال ليث يعني ابن سعد حدثني بكر يعني ابن عبد الله بن الأشج عن نابل صاحب العباء عن عبد الله بن عمر
مررت برسول الله ﷺ وهو يصلي فسلمت فرد إلي إشارة وقال لا أعلم إلا أنه قال إشارة بإصبعه
Its isnad is sahih. Nabil, known as Sahib al-Aba, is a reliable narrator whose hadith is recorded in the Sunan, and Nasai and Ibn Hibban confirmed his reliability. Abdullah bin Umar is the well-known companion. The hadith was narrated by Abu Dawud, 1/243, no. 925, in the book of prayer, chapter on the greeting of peace during prayer, by Tirmidhi, 2/203, no. 367, who graded it sahih, by Nasai, 3/5, no. 1186, by Ibn Majah, 1/235, no. 1017, by Darimi, 1/364, no. 1361, and by Malik, 1/70, no. 81.